I haven't seen direct, crude, deliberate cyber-bullying in the sense of going after one particular individual from thread to thread, or else I would report it.
But there's a lot of subtle, unpleasant, intimidatory behaviour which goes beyond being frank, witty, forthright, or anything else that I enjoy about Mumsnet. I believe that most of this is unknowing, and that individually many of the most aggressive posters are perfectly likeable people offline. But the sum of the whole ends up being a lot worse than the individual parts.
There are bullying dynamics:
Posters being very aggressive and intimidating less confident types into silence, then claims being made that aggressive behaviour is justified because it represents the majority.
Status being gained by being witty in a cruel way at the expense of others who are weaker (in this forum, being verbally dexterous and socially sophisticated plus having a thick skin is strength)
People being given the clear message that they do not have the right to complain or object or else they are prissy, stupid, wimpy, whingy, and grassing (reporting)
Discrimination against people who fail to measure up to standards of the group - which amounts to class discrimination much of the time and certainly intellectual snobbery. I've lost count of the number of times posters have been sneered at because they were less articulate or sophisticated in their expression, because they weren't net-savvy and didn't know that you need to lurk before you leap and get a feel for the tone of the forum, because they allowed their wounded feelings to show before adopting a suitable attitude of detached curiosity towards the responses.
It kind of reminds me of the girls at school who used to sneer at people for wearing the wrong clothes or seeming a bit too babyish or being awkward in some other way - only on here it's all verbal.
Another bullying feature is the sharing of flippant responses and in-jokes between certain posters in a way that effectively excludes and belittles the OP - a clear message that for whatever reason "you are not important enough for us to take your feelings and opinions seriously, and you're certainly not one of us."
And above all I resent the suggestion that you should not post on mumsnet if you are unable to take any of the above behaviour on the chin, switch the computer off, namechange, and forget about it.
In other words, if you are socially isolated, desperate, frail, sensitive, insecure, mixed-up emotionally, have low self-esteem and therefore brood over insults, or generally vulnerable and needy, this isn't the place for you, even though it's a parenting support forum??
And if you have obviously ended up on AIBU not knowing what you have let yourself in for, then you deserve what you get?
Fortunately this is NOT the majority of Mumsnet which remains a fantastic site, and again, I am talking about a negative dynamic not individual posters being unpleasant people.
But I do worry that it is spreading, and I also do think it is important that people who are uncomfortable with it keep speaking up about what they see, if only to get a more balanced portrayal of what Mumsnetters actually think.
I certainly prefer it to reporting people, which I haven't yet done because as I keep saying (tired and struggling with words, sorry) it is mainly something that happens and people get caught up in, rather than being the direct product of any one person.