The difficulty most people have in understanding online social networking is that they view websites as having a specific purpose, like CNN.com for news and YouTube.com for videos and Flickr.com for photos. But social networks like MySpace and Facebook don't provide a specific use so much as a they provide you with a home on the web.
Think of e-mail as a P.O. box on the web. A P.O. box is a very handy way of receiving letters from people, but you aren't going to invite people to your P.O. box to look at your family photo albums. You are going to invite them over to your home.
That is what an online social network provides: a small piece of the web that you can call your own. You can use it to keep up with a blog, to let your friends and family know what you are up to, and even to share photos from your recent vacation.
And because a social network does most of the heavy lifting for you, it is much easier to maintain than a personal website.