An online diary that's good for writing down day-to-day events, and keeping in touch with friends. LJ makes filtering posts simple, so it's easy to write whatever you want and share it with people you have an acceptable comfort level with. Very spiffy for letting off steam and putting it in a box. The community feature is a nice touch. I use the uwaterloo one and people there are helpful and responsive. Seriously, if I ask about - say - a professor, I'd get replies within the hour.
Recommended Use: Journaling - especially for drama and angst. Keeping in touch with faraway friends or family (hi sis!)
At first, I tried using this like LJ to keep in touch with friends. But honestly, it's a major pain: you'd need to bookmark or link to your friends' blogs, whereas other places (see Livejournal, Facebook, Twitter) keeps all friends update on one page. What most or all blogging tools *will* do for you is give you freedom to customize your page. It's good for publishing your own content - writings, thoughts or news.
Recommended Use: Decently long thought-provoking and-or humorous entries. Or pseudo-intellectual, pretentious writing. The latter sums up my blog perfectly!
Facebook needs no introduction and everyone knows its many tricks. However, there are ways in which people are doing it wrong. In addition to what that dude said, I personally find LJ better for keeping track of friends: less cluttered and easier to read. Other than that, Facebook does many things well - which is why it's the market leader in social networking and killed off crappier attempts like Friendster.
Recommended Use: finding old friends, contacting people, organizing group events, sharing photos,
Same as LJ except for people with ADD. Or likes constantly receiving updates on mobiles. Useful for tracking/talking to celebs. 140 char lim
# StumbleUpon [echoblaze]
As mentioned earlier in the facebook entry, StumbleUpon is good for discovery cool things on the web. If you have time to burn, you can continuously click that "Stumble!" button and be forever amused by the vast expanse that is the intertubez. Or if you have a lot of interesting content to share (i.e. whoring your own stuff), then you could attempt to push it here.
Recommended Use: killing time
Social bookmarking. You upload your bookmarks and share it with friends or others; you could check out cool stuff by looking at the most bookmark-ed items ever. To be honest, I didn't use StumbleUpon or this much because I don't have too much time to burn. Also, if you start having tons of bookmarks - it's honestly easier to search for what you want in Google.
Recommended Use: killing time
# LinkedIn [Emerson Cho]
I'd call this "professional networking" as opposed to social networking. On the recommendation of my coop advisor, I went and played around with this toy today. Functionality is similar to Friendster - the coworker recommendation system is similar to ye olde testimonials, but there's a lot of extra structure in place: job searches, sorting by companies or fields of work, finding coworkers. Not sure whether this will pay off - I find it more likely that people will add to their professional networks IRL. But if nothing else, it'll be good for keeping track of ex-employers and ex-coworkers.
Recommended Use: too early to tell. I suspect it will become an e-rolodex.