Facebook has recently integrated the Developer's Platform into its core site, allowing users to choose from over 200 applications.
Among the most popular:
-Top Friends: Choose up to 24 friends to display directly on your profile, allowing you to both a) play favorites and b) shorten the number of clicks it takes too check up on your best friends.
-Free Gifts: Don't feel like giving to charity? Give little gifts (represented as occasionally clever icons) for your friends to display proudly on their profiles- for free!
-Video: Yes yes, now you can post videos on your profile also! Or send video messages!
-Flikster Movies: Share what movies you've seen recently with your friends- great for recommendations.
-iLike: Add music that you've been playing through iTunes. It also lists upcoming concerts, allowing you to see who else may be attending.
-Trakzor: This only works if your friends install the application also- but you can see who is viewing your Trakzor page. Note- stalking is still a 100% safe and viable Facebook activity- if you don't want to be tracked, simply don't visit people's Trakzor pages.
My personal favorites:
-Graffiti: Placed right above a profile's Wall, allows friends to draw on your wall.
-Games: Play a Battleship-type game (using sushi as playing pieces as opposed to ships), strip blackjack (they censor out the genitalia- rated E for everyone!), and more (play tetris while you wait for other users to join your game- either random or from one of your networks).
-Honesty Box: Allow your friends to anonymously tell you exactly what they think of you (don't worry, it's private- you can revel in your shame/anger/surprise in peace!). However, like the Trakzor app, your anonymous commentator also needs to install the app in order to send you a message. Once again, a useless application unless/until it becomes popular.
-(fluff)Friends: Add little graphic friends to your profile (mine is a flying giraffe I named Sasha).
-Extended Info: Add more info boxes to your profile (I added "Things I Am Looking At Right Now" and "Summer Schedule").
-Wis.dm: A Q&A site, Wis.dm's Facebook app asks questions of users, pairing them to other users who've answered similarly. Users gain points as they answer questions- a veritable smorgasbord of possibilities for advertisers.
MySpace is working on a news site, currently in Beta: http://news.myspace.com . I especially like the integration of Event news from LinkedIn and MeetUp. They've also recently developed a profile customizer utility that's incorporated within the site, removing the necessity of seeking external MySpace Layout sites for the non-HTML-savvy.
Also of note: For some reason or another, the number of pornography accounts requesting my friendship is exploding. At least they are easy to swiftly detect and decline- a game of "spot-the-thong".