Here's how I use them.
Facebook is for friends and family. I have my own business and can go for days without talking to anyone except DH and cashiers. FB is like my virtual office. There's the newsfeed where I can interact with everyone, even people who I might not hang out with outside of work. Many of my FB friends are ex-coworkers. There is also the inbox where I communicate privately with my "real" friends. Before I started my blog FB and these boards were all the social networking I needed.
A Blog is like an online diary. I never thought I would start one but for some reason I came up with this crazy idea to cook with my solar oven every day for one year and blog about it. Many people start blogs as a kind of family scrapbook. I wanted my blog to be seen by as many people as possible, partly because I'd like to promote solar cooking and partly out of vanity. For this reason I started using Twitter. Mostly as a tool to drive people to my blog. And it worked. Just today a received an email from the president of a company that make solar ovens and he wants to meet/work with me.
Twitter is the hardest one to understand. I really don't get how anyone can follow millions of people. I get new followers every day on Twitter now that I have a specific thing to tweet about (solar cooking). I didn't get any, except for porn spam, with my personal Twitter account.
Hope that helps. I'm new to all these things too.