Other sites like facebook were created for anybody to join.
Facebook was created with college students (American colleges, i.e., what in the UK are more commonly called universities) in mind.
You signed up with your uni email address, and became part of that uni's network. You could then browse other people's profiles on your network, if they let you do that (you could by default).
Then, a year or so ago, facebook opened the site up to everyone, even if you weren't at university. Unfortunately, the model they've had since the beginning is very much set up for uni students.
This includes networks. You can get regional networks, for example the Manchester network, which you can join and then be able to see many people who are also in the Manchester network.
Unlike similar sites like Myspace, it's really made for keeping in contact with current friends, and not so much finding new friends online.