There are many social networking sites out there and they are all different.
Facebook is probably the most popular one right now among all the sites but some of them may serves different purpose. Facebook actually have great appearance, easy to use, and advertisements that are small. Updates seems to be very often with Facebook, but things are still easy to look around. Facebook changes a lot and keeps on added new stuff like the timeline and developers with Facebook apps.
Unlike Facebook, LinkedIn which deals mostly with businesses professional and jobs seeking. Unlikely to find any social photos like the one in Facebook, people only show bio picture and information about their career. It is easy to browse around too but more ads so a little bit annoying.
Twitter is somewhat different than the others, Twitter users use Twitter to share short messages called tweet and is limited to 140 characters. I think Twitter is annoying if you have lots of tweets or following lots of users because the tweets jam up and it feels like too many to read. One more bad thing about Twitter is many businesses use it to advertise their business and updates so most the tweets is not personally related.
The fourth social networking site is Myspace. I think this one was the worst one out of the four because first it has large and something like a movie clip ads, then there is no uniqueness to the site. Myspace was so popular before but gone down hill after Facebook. I saw how Myspace was before in the old time, but now it is totally different and looks like they copied Facebook's format.