For socialising you don't really need it, like people have said you can make friends by meeting people in person
However, I find it very helpful for studying. Most of my classes set up a Facebook group so we can all talk to each other about things we're finding hard, coursework and sharing helpful websites for revision. For example, there was a delay in uploading some of our lab results for a practical write up so everyone just posted there on the group so we could get started earlier. One time our lecturer even joined the group to answer any questions we had! I find that really helpful.
Also don't worry about people thinking you've only just made a Facebook account. I made a new one for university because I wanted a fresh start from all the embarrassing things I posted using the one I had when I was at school so I only have a few friends on my Facebook. No one ever said anything about it to me - how often do you look at how many friends someone has anyway?
Could this happen to you?