I have wondered this myself. That's why I'll never reveal my absolute favourite name on here. If another poster suggested the name to anyone I'd probably do my best to dissuade them from using it too!
I think that Mumsnet does make names more popular because IRL you have to be polite about people's name choices, regardless of personal opinions.
Online you give your true opinions. If a quirky name seems well-liked online, than most people probably like it in reality. Therefore you're more likely to use it. It will become known to more people, so more people will consider it as a name option. Then, as most people like it, the name will rise in popularity.
My explanation probably isn't the easiest to understand. It's difficult to explain in writing! Hope you get the gist.