At the risk of getting all philosophical like Batty just before he tries to kill Deckard "I've seen things..." or Cosby-like cantankerous. In my 18 years of using the internet I have had a lot of ups and downs. There is definitely something about my online persona which some people take an instant dislike to as I don't have anyone trying to fight me in real life.
It is good and bad that you can hate people you have never met and completely negate their existence by blocking them on Facebook.
Although I started using the internet in the first years that the web took off, my first introduction to talking to people over the web was through USENET (http://en.wikipedia.org/wiki/Usenet) and also on a tv discussion group that I managed to get a letter from a major media organisation sent to the college IT administrators and temporarily lost my network access while I wrote a letter apologising to the head of the network. The plus side is that I met one of my best friends through it and still talk to them until this day.
USENET even turns up in the first Mission Impossible movie "SCROLL USENET GROUPS" shows the list starting with alt.alien.vampire.flonk.flonk.flonk.
The group I posted on the most throughout that time was alt.religion.kibology, which is nothing like a religion and does have the majority of its users from the USA with people all around the world. A rubber chicken was mailed around the world via the group at one point. I did send a large amount of chocolate to a person in Toronto after she had just been released from hospital and her boyfriend hugged her so hard she had to be re-admitted due to six cracked ribs. Tumbleweeds blowing through the USENET group these days, but it still exists in a smaller form on Facebook and I am friends with a lot of the people from it.
As for the bad experiences, I did try and engage with the SomethingAwful forums, but it left a bad taste in my mouth after everyone piled on in one thread. I only post one thread there per year generally and reply even more rarely. You do really have to watch your bandwidth if post images/video to the site. I have 100Gb of bandwidth per month for my website and people looking at four animated GIFs I had hosted on my website used up 80Gb in four days.
I don't want to name it, but there is a local music forum that irritates me, people hide behind usernames and have even gone as far to make entire threads to annoy me. They still suck up and want photos though, I can see the visitors coming to my site from there.
Facebook is good and bad in that the messages do not come with a subject line so you do not know the content until you click on the icon. When people sent emails without a subject line it is usually bad news. Also the hivemind aspect to Facebook can get quite annoying at times.
I don't really use Twitter as much so I can't really comment on it. I at least post different things to Twitter than Facebook and if you have a bad experience on it is your own fault as you can choose who to follow.
At the risk of bringing down the wrath of the Iron Fist, I am going to mention the Cave Clan forums. You can't get in to them unless you go through a probationary period although that is more for exploring. I joined when you still had to join by a sending a letter to the PO BOX. I am too busy to go exploring now, but it has a large contingent of photographers and its own film festival these days. Due to people confusing what sort of "underground" they are, they do have the occasional problem with people who obsess over them. There is a lot of bagging out people which I don't really support but I still go to see them for the anniversary at the pub each January.