Yep. I became involved with a online fandom associated with a very popular sci-fi programme. For the first couple of years it was great, I moved from the online forum to socializing with local fans...I thought I'd made some good friends.
Then I started dating my now partner, who I met through this fandom. One of the local fans took exception to it - they had briefly been involved with him a couple of years previously for a couple of months, but it had died a natural death - but they were still very possessive and didn't like the fact my partner had moved on with me.
It all got very nasty, very quick. This person did their best to turn the local group against me, by spreading all sorts of nasty rumours...and I was shocked to see so many people believe them. I learnt at that time that a lot of people will believe the first version of a story that they hear, and it's very hard, if not impossible, to change their opinion going forward.
It even got as far as this person and a few of their 'friends' setting up a fake twitter profile, pretending to be me. They then posted a lot of defamatory stuff, dragging in cast members of the programme itself. This was over 10 years ago, and what I regret most now is that I didn't take it further by reporting it to the Police etc.
Myself and my partner ended up withdrawing completely from this fandom, cutting all contact with all that were involved. Out of a group of about 30 people we were left with 2 that we still consider friends....and to be honest, if it wasn't for my partner, I would have left them behind as well.
I went to public all girl schools for the whole of my education. I experienced nowhere near the sheer ugly bullying during that time than I did when I was in my late 20s from this group, it was a real shock to me. I learnt that most of the time there is absolutely no point trying to fight this sort of thing - it's a bit like politics, you are never going to change someone's mind for them. Best thing to do is get as far away from these people as possible, and stay away....live the rest of your life like they don't even exist.