I haven't read Fangirl yet, just an extract online, but I wonder if Cath having a twin influenced how she relates to people online. If she already had someone to squee and flail and speculate with, she might have been less likely to develop personal relationships online - particularly if she only posted on forums but never chatted one-on-one with anyone. Or swapped email addresses or added them on social media, etc.

From the tiny bit I've read of Fangirl I could imagine that Cath might be shy about initiating private discussions with people... and not feel a strong need to overcome that, because often it's just easier to go talk to one's sibling.