How else do you befriend people?
I like to try talking to them in real life first.
Just seems slightly weird/stalkery to go and talk to someone online if you have absolutely zero relationship with them in person.
Personally, I don't accept Facebook invites from people I don't like or I don't know. (Which includes people I don't know beyond the fact that they exist).
You should talk to them in person first. And if you can't get up the gumption to just out-right talk to them, because you may feel that it seems equally odd, then come up with some silly reason to talk to them, like by asking them a question about say, something in a class you share.
If you can get yourself on the terms of being acquaintances, and you both have a reason to talk to each other, such as discussing class work, then you can actually start making a relationship from there.
