One of the best social networks I've ever had the pleasure of using is FriendFeed. At first glance, this site looks an awful lot like Twitter. But it's so much more than that. FriendFeed aggregates statuses, comments, messages, and posts from dozens and dozens of sites and puts them all in one place. By checking your FriendFeed profile, your friends can see updates from your Facebook, Twitter, YouTube, Flickr, Picasa, GTalk, Digg, Delicious, Amazon, StumbleUpon, and Google Reader accounts, as well as many more. Plus, there's RSS support, so you can share your latest blog or podcast entries as well. Plus, you can have all these entries published to your Twitter, so your friends there can stay up-to-date with your lifestream. Personally, I would prefer it if everyone on Twitter would move over to FriendFeed. For one thing, the site is much slicker and more useful. The page auto-updates instantly, so you never need to refresh it. Plus the site is much more stable. I've never known it to crash, while Twitter crashes on almost a daily basis. Finally, FriendFeed was just bought by Facebook (FaceFeed?) so hopefully more improvements will come.
A really neat site that I just joined a week or two ago is gdgt. I think gdgt is really interesting because of how different it is. Members browse or search the site for gadgets and gizmos that they currently own, used to own, or wish they owned. When you find something that falls into one of those 3 categories for you, you add it to the appropriate list. Your profile consists of those three lists, as well as any other information you choose to share. There are also forums, a news page, and a feed of recent member activity. I think gdgt is really pretty innovative because I've never seen a site quite like it, and I must admit there's something almost addicting about looking for anything I own that might be considered a gadget and seeing if it's listed. If a gadget isn't listed, you can add an entry yourself, but there are still a limited number of categories because the site really is brand new. So far I've got 15 Haves, 15 Wants, 1 Had, and made 1 new contribution. Definitely check this one out.
There are also a ton of sites you can use to supplement Twitter. Twitter is my favorite social network by far. I've met hundreds of new people on Twitter, a handful of whom I consider close friends, even though we've never met in person. Some great sites that make use of Twitter include Dailybooth, TwitPic, and Blip.fm. Dailybooth is sort of a Twitter for self-portraits. Each day, users snap a photo of themselves and upload them. Links to these photos are sent out on their Twitter accounts, where their friends can view them and leave text or photo comments. It's a fun project to try and take 1 picture of yourself every day while keeping it fresh and interesting. TwitPic also auto-tweets links of uploaded pictures, but it's more of an all-purpose site. Use DailyBooth to take a self-portrait each day, but use TwitPic to share something funny, interesting, or just cool that can't be described in 140 characters. Finally, Blip.fm indexes links to MP3s and YouTube videos of popular (and obscure) songs for your listening pleasure. It auto-tweets a link to each song as you listen to it so your followers can listen along with you.
Posterous, while not inherently a social network, has some very cool potential social networking applications. At its core, Posterous is a blogging service. But what sets it apart from other services is the fact that you post blog entries via email. In fact, you don't even need to make an account to start using the service. All you need to do is send an email to their address, and the contents of that email will become your first blog post. The neat part is that if you send audio files as attachments, they will be uploaded to the server and a flash audio player will be generated. This allows for easy audio blogging and even podcasting. The social networking aspect of Posterous is that it allows you to auto-publish links to your entries on any number of social networking sites. This means that by sending a quick email, even on-the-go, you can share virtually any kind of content on Twitter, Facebook, FriendFeed, or whatever other site you like, have all your friends and followers view it, and keep an archive of it all. How great is that?!
There are so many other sites I could list here, but I've already rambled quite a bit, so I'll just end by saying that if you want to see my updates on all these sites and more, you can visit my Google Profile.