People like Lolo or Symmonds are really outgoing people who like to interact with the fans. They seem to feed off of people and (being honest) the adulation they get from that. Rupp comes across as really shy, which seems to be why he doesn't do much on social media. Maybe Salazar has something to do with it, but at the same time Mo Farah is active on both Twitter and Facebook, so who knows.
I do agree, though, that he should have some more of a presence. It's just good business. Carl Lewis said in the 30-for-30 that he took acting classes to become more comfortable talking to people and doing interviews. Rupp's career is going to end someday (hard to believe he is 27 already) and if he wants to make any money through his running after his career, he needs to do better.