MySpace vs Facebook
I've had a MySpace account for a while now, although I only just started using it because I found out a few of my friends were on there. Facebook, on the other hand, is something I've known about for a while now but I never bothered to sign up until a couple days ago when a friend sent me an invite. I've had a chance to play around with both of them for a bit now, although not in any in-depth fashion, so I thought I'd go through my thoughts on each.
MySpace
It's been around a while and is possibly still "cool" so there are tons of people on there. I didn't realise until tonight that one of my best friends from high school (whom I have mostly lost tough with now) has an account. If you just want to sign up and becomes friends (as in MySpace "friends") with a bunch of people really fast then MySpace is probably right up your alley.
I want to state right now that MySpace has possibly one of the annoying bugs – at least for me – that can occur on the internet. Despite the fact that there's a "Remember Me" checkbox when you log in (which is always ticked for me), it never actually remembers me and I have to log in each time. Furthermore, if I'm forced to log in in the process of going to a page (such as if I want to read someone's blog post) then I'm not taken to that page after logging in but rather to my home page. Why? Things like this are the reason I think MySpace is so poor put together.
The look and feel of MySpace is really pretty awful. The default theme is rather inconsistent and lacklustre, which is probably the reason so many people "pimp" their profiles. I am one of those people. Attempting to do this pimping yourself, even if you're a web developer such as myself, is an exercise in frustration. For one thing, you're not allowed to place your own CSS stylesheets in the head area of the page but rather you have to put it in your "about me" area. This causes a flash of unstyled content (FOUC) where you have to page display in the default theme for a second before your own stylesheets kick in. On top of this, the page layouts are based seemingly entirely on tables and don't use any sort of hooks, so attempting to style a certain area requires lots of trial and error.
While MySpace does provide a few features, I find most of them fairly useless – but this is probably just because I'm a bit different to the target audience. I find the blogging capabilities woefully inadequate, for example, but I run my own personal blog which I can tailor to my liking … so comparisons will obviously fall short. MySpace really seems to adopted an "old skool" style of site, with various areas being blogs, forums, mail, bookmarks and so on. Some of the new areas are things like video, which looks like sort sort of attempt at competition with YouTube.
Overall, MySpace is fine if you just want to check out who's online, leave messages for your friends, run a basic blog, show off some photos or post on some general forums. It's not a nice application to work with for an extended period of time though and Facebook, which is admittedly a newer competitor, is much nicer to use.
Facebook started out an application aimed at building college social networks. It has since expanded to include general social networking, similar to MySpace. It turns out plenty of people I know have Facebook accounts although this is understandably skewed towards people I know at uni. If you're just wanting to find people online then Facebook fulfils this need quite well. When you sign up it asks for access to your online email accounts (and can handle desktop email programs like Outlook as well) and will use these to find out which of your contacts have Facebook accounts as well so you can send friend requests immediately. I don't care for this procedure too much as I'm paranoid about handing out my username and password details. I did use my Hotmail account as I just use that for filtering spam anyway. 
Overall, it's still pretty easy to search for people you know and then even easier to find people that are linked to them.
The look and feel of Facebook is pretty good. The look may be a bit basic but it's pleasant enough and doesn't start pissing you off after extended use (I count this as more than an hour on the site). Navigating around is easy and there's plenty of bling in the form of dynamic content. For example, items on your profile can be moved around the page just by holding and dragging their title bar. Similarly many of them can be removed by clicking a small cross in the title bar. Quite unlike MySpace you are unable to edit the look of your profile at all. Personally, I'm not too fussed about this. The Facebook theme is nice and simple and not having the option of changing the theme means I don't spend any time trying to make it look better. Plus it makes writing applications easier (see below). You're also asked what networks you want to be a part of. Initially I think this was just for which college you went to but it's expanded to include what country to live in or were born in, what company you work for and so on.
Some of the Facebook features are similar to MySpace, such as the "friends" concept, photo galleries, groups and displaying personal information. Facebook, on the whole, takes these a lot further than MySpace does. For example, photos are divided into photos that you placed on there and photos other people have put up that have you in them. This distinction is achieved by allowing you to tag photos with the people in them. This process is quite easy as you just click the face of the person you want to tag and either type in their name or check a box for their Facebook profile. Other people can then see who the people in a photo are by moving their mouse over the faces. As another example, personal information that you type in comes up as a bunch of links in your profile which you can use to search for other people with the same interests, hobbies or whatever.
Most of the things you'll want to use in Facebook are examples of applications. Applications are scripts that use the Facebook API to integrate into the site. The photo and groups functionality are both inbuilt kinds of applications. Examples of other, custom applications are a friends collage app, a "what I'm listening to" app and a "cities I've visited" app. I've got a Sorting Hat application on my profile at the moment. Down the track I might look further into how to create an application as I'm sure it'd be an interesting experience making one. Any ideas for something I could make?
In terms of communicating, Facebook has applications for leaving messages on people's profiles (on their "wall"), quite like MySpace. You can also view feeds of updates on what your friends are doing on Facebook (adding other friends, changing their info, joining groups, etc), leave comments on photos, post on group walls, see what's going on in a network and a ton of other things that are too many to list. One thing I would like to draw attention to is the "poke" functionality. Poking is a generic way of communicating in that it just tells someone they were poked (and they can poke back or whatever); the Facebook designers left this deliberately vague so people can work out their own meanings associated with poking. It does look kind of weird though when you see, say, "Poke Chris!" on a page. Oh, and I found out you can "Poke Yourself!"
Overall, Facebook is a pretty interesting site. There's so much you can do there just by adding applications that the possibilities are staggering. I try to limit my time on there as I found that once I started digging through the things you can do you can get lost in there!
Final Thoughts
I'd say Facebook is definitely the better of the two. It's easier to use, better put together and more generally customisable. It also has a bunch of nice features that make interacting with other people ridiculously easy. Of course, there's nothing stopping you from having accounts on both (as I do) so the socialites among you can get your fix at both places. Personally, I'm not much of a "social networking" person but I do like browsing around the sites and seeing how things are structured (although MySpace saps my patience fairly quickly).