You don't come across well here, tbh.
Whether you have an existing complaint about any manager doesn't negate the fact that you have commented about your employer/colleagues on Facebook (and negatively, to boot).
When will people start getting that this isn't acceptable?
I'm sorry, OP, that you are feeling upset and bullied - that's not on. But it is never acceptable to post comments about your professional role online, or, at least, never in the last 10 years from the contracts I've signed - I remember when I got my first job in (my current) industry this sort of stuff would be covered under the "acceptable IT usage" policy, but my previous employer x2 had comments on websites written into a different section too. My current employer specifically lists "websites and social media platforms" from what I recall. And since I signed that 2 years ago maybe they've altered the new ones to mention Facebook, Myspace etc by name now, for all i know.
Anyway, if you don't get sacked this time, count yourself lucky - many haven't been so fortunate. My own organisation is quite large, but the office I work at only has space for a maximum of 40 - 45 project employees - and I know of 2 cases in the last year where people have been fired (1) or faced a diciplinary warning because of crap they've put online (the other one).
I also know for a fact that my boss has not hired people because of stuff he's found online about them. Not officially, of course - "stronger candidates, etc." but I know what goes on in private.
Good luck.