Facebook - how do they know!

Hi all, I'm not sure of you are still hanging in there but I didn't get finished so I am still plodding on. I have just joined face book. I ony have one question: How do they know? I was immediately bombarded with a list of people, several of whom i know (that's the bit where the question applies), and plenty who I don't. I only want to use it to talk to some people where that is the only way. Generally because they live in another country. And even then, now that I 'skype' there are limited uses for Facebook. So i will see how long I stick with this...