Have to agree with every word gin says. When I first discovered MN I couldn't wait to jump into threads and join in, I loved the fact that lots of people were busy chatting and having a laugh.
I found it welcoming and enticing, not off putting and cliquey. So I pulled on my glittery wellies and waded in up to my hips 
But IRL I have always been very sociable. Always liked big groups. My motto has always been 'the more the merrier'.
I don't have any time for actual cliquey bitchiness or trying to deliberately exclude someone. That has never been how I get my kicks. Not at school and not in my adult life.
I think the sort of person who dislikes these free for all chatty threads, and feel snubbed and excluded are probably the same person who stands at the school gates glaring at other Mums chatting, and thinking 'Snotty bitches, all ignoring me'. Rather than, you know, smiling and making eye contact and getting a conversation going themselves.
No one is such a special snowflake that everyone is going to run up to them with open arms the minute they step onto MN (or into the playground).
They really need to get over themselves a little bit. Stop feeling sorry for themselves and being begrudging...and start joining in.