Craigslist is only SORT OF social media, but I bought a large chunk of Tony Vincent's guitar and recording rig from him last spring, and he was wonderful to work with, incredibly helpful and supportive along the way.
Also, Alice Ripley is great to talk to every now and then (we share the same birthday).
I was friends with Anthony Rapp on social media for a while, but things got strange pretty quickly, and I believe his boyfriend at the time may have been impersonating Anthony or something? It was weird.