Pseudonyms

I've been on the internet and forums of one kind or another since the 1990s. In the early days, everyone used a nickname, especially women. And you made sure it didn't sound feminine if you didn't want trouble. There is still a lot of that thing going on with social media, but it is nothing new. On Facebook, I had to use my own name, it felt very odd and unsafe.

Thank you. I thought that was probably it. I don't think I mind being recognised so I shall stick with my name.

As MissAdventure says it's to avoid recognition. I don't use my real name any of the forums I belong to, whether they are public or not.
Although my user name is an actual name it's not the one I'm known by in RL

Well, the forums are open, meaning that anything you write can be posted elsewhere: Facebook, Twitter, the dailymail, just for starters.
Its probably not advisable to write your full name, unless you're happy for the world, his wife and his dog to know all about you.
Even then, there have been people whose details have been recognised elsewhere by family members.