My ex and my current partner know I use Mumsnet because I often mention it.
I think I've even quoted some Mumsnet threads on my Facebook before, so no doubt my family may also know I'm on here.
But then again, I can only really think of 2 people who cba to actively seek me out on here and they're fairly obsessive gossipmongers themselves (my Mum and one of my sisters), so yes, I do try to be careful what I write.
But sometimes, you just need to tell the whole story to get the right advice and opinions.
As always, once you put your first online footprint down, you're trackable forever thereafter, so start as you mean to go on.
I'd like to know how to have a thread deleted, that way I can post about relationship issues and after a few hours delete it in case my boyfriend has been looking. Then again, I know that he was very active on a work related but quite social forum a few years ago and was a moderator too I think, yet I've never felt compelled to search out his posts. I suppose I wouldn't want to come across any cringeful stuff (which we are all guilty of) or flirting.
It's never occurred to me to seek out people I know who I think might post on here, but I'm pretty sure I'd know which boards they'd post on if I did. That's not why I'm here - snooping - that's what Facebook's for! 
I'm here for personal advice and to gain insight from other threads.