Yes, even my parents are on facebook. There are 2 reasons for this, which probably show you what facebook can be used for ...
1) My dad's family tree research - we don't just get people we were at school with asking to be friends with us, we also get random people with the same surname. It's the joy of having an unusual name, and it helped persuade my dad to join.
2) My whole family were visiting my parents. My sister fell asleep in a chair and my brother took a very unflattering photo of her. He had posted the photo on facebook before my sister had woken up. (I'm very glad I'm the oldest.) My mum thought this was very funny.
Personally I only created an account for file sharing purposes. I continue to use it because I am a very nosy person and every so often enjoy seeing what my friends want to tell me about their lives. I also arrange to meet friends with it, as it's an easy way to communicate with a group
But I spend much more time on MR.