Facebook?

Okay...so I was slow to the world of blogging and am still not the best blogger out there...but I participate and use it to let people who care know what's going on with me and my family. However, I don't get Facebook, MySpace, Twitter, etc. What is their purpose? Are they blogs? Why do people keep inviting me? What would I do if I decided to join Facebook? Would I even have anyone follow me there? Maybe so since I keep getting invited. :) Can anyone fill me in? What's the difference between a blog and Facebook?