I understand why you're feeling like this as I can imagine myself feeling and thinking exactly the same way. I have an intense fear about crossing a line and being told 'this is the last straw' even though logically I know this won't likely happen.

Could you try and address this with T more generally without mentioning Facebook? You could bring up the fear that you'll cross a line and he won't work with you. I've mentioned this to my T before. You could explore the fear together without delving into anything specific. It sounds especially important to talk about the bad feelings towards yourself.

Alternatively, would it help to have a 'plan of action' to explain the accidental friend request if it ever did come up (I think it's so unlikely)? Maybe say that this person has a similar name to an old friend who popped up and you accidently clicked on the wrong person. Something like that.

I really feel for you as I know this is horrid. If it helps, please know loads of people look up their therapists and their relatives/friends on Facebook. It's really quite a common thing. If a therapist has a Facebook page they are going to expect it to happen at least once in their career, probably many times.