Why do people do this?

[Jokkenson]

On Facebook (I don't know why I am "obsessing" over this) why do people sen me a friend request or if I send them one and they accept it, and I try to talk to them like just a simple "hey", they ignore me. Most people on my friend list I actually know (I may not talk to them very much or at all, but we actually know each other.) Like I messaged someone and they had to ask who are you even though they finally remembered me and they asked why did I message them when I just said "hey". I know theres that one mutual friend competition where people compete to get the most mutual friends but why do they accept my friend request or send me a friend request If they just ignore me? All I say is hi or hey not something stupid or whatever...