I'm told that being involved in a hundred different social media sites will help my (very) small business, but the truth is, you can't spend all day online promoting on a hundred different sites. Pick a few that you can keep up with consistently and let the rest be. The most important thing to remember is that the sites you use are tools. Simply joining Facebook doesn't mean you'll automatically get 500 fans and lots of sales or business traffic. You have to use the tool to bring people to you, just like anything else. Because of that, make sure you really enjoy the few you choose to use.
There are tons of different social media sites - Facebook, Blogger, Twitter, Squidoo, Flickr, Pinterest, YouTube, and StumbleUpon are just a few of the most common.
To conclude - I have to reiterate: enjoy what you're doing, or it's a waste of your time!