why am i so bad at maintaining friendships?

i've never been very social growing up, and upon entering college, decided to try to change that. i'm not bad at chatting people up, and getting along, but we always end up exchanging numbers, and then we never talk again.

now i know the obvious answer is to text them every now and again, but i don't know how to go about doing that. what is there to talk about? just saying "hey" with no context feels weird.