‘If I leave Facebook I’ll lose my friends’ is only a rational concern if a) you subsequently fail to communicate with your ‘friends’ in another manner or b) you have ‘friends’ that prioritize using Facebook above sharing life with you in same other way.
Losing friends because you’re not on Facebook is only a valid concern to the extent that your ‘friends on Facebook’ are not really friends at all.
A more interesting question: who are your friends when your social life isn’t dictated and monopolized by a big, nasty advertising company that literally makes money by addicting you to feelings of rage, indignation, inadequacy, jealousy, and meaningless push-button approval? That, to me, seems like the more pertinent and pressing concern.
Facebook, in fact, has nothing to do with friendship.