The first ever that I have come to know was friendster and that was in the beginning of January 2004.  Back then, online community was just an innovative and more fun ways of keeping in touch in place of emails and ecards. Then came mySpace, Facebook, hi5 and so on and so forth. I tried to maintain a profile in each site and I realized it's the same friends I'm hanging out with but just in a different site. So I stick with Friendster (FS) but ocassionally checked other sites for messages and updates from a couple of friends who are not on FS. In time, it became boring--at least for me. I stopped messaging my friends but just the thought of having them in my friends' list is more than satisfying. At least I know where to find them and that's what matters most.

The next best thing that cropped up was the photosharing.  Now you don't have to go through the tedious job of attaching of photos in your mails which used to take so much time especially if the files were big and you don't know how to resize them first before sending them. (Speaking from experience LOL!)

As time went on, we see yet more improvements like pimping our profiles, adding videos and music and so on and so forth. It's getting more and more complex yet so simple to use because everything is done with just one click of a mouse.  Then some more applications become available like photo slide, games, and about anything one can imagine to make social networking more fun than ever.

I think that whatever your reason is for using any of these sites is the indicator of whoever is the best for you.