If you ran into someone on Facebook, as in, you saw there post because they were a friend of a friend or something, and you hadn't seen them in like 2-6 years, would you send them a friend request?
It really depends on the situation. If I have been on good terms with this person and/or have a decent history with 'em, then why not? I probably wouldn't if they were a standard acquaintance and did not exactly "mesh" with them in the past.
I've ignored childhood friends who sent me friend requests. I just have no desire to reconnect with anyone from my past. There are two many things about my life that I would have to explain, and I just don't feel comfortable doing that.