When someone sends me a link request on a social network and I haven’t met them personally, there’s one key way to determine whether to accept the request: review who else we know in common.
Facebook and LinkedIn make this especially easy. If I see a request from someone and it turns out we have 10 friends in common, yeah, I’ll accept the friend request. We’re clearly part of the same network anyway, so we might as well make it smaller. Similarly, if we don’t have any mutual friends in common, I’ll probably turn down the request.
It is very much about who you know – namely, who you know in common.