As I usually do, I start off my mumsnet ting with a quick look at active convos. All of them (all 100 on the list) say last post was on 16/12/11 
If I click on a particular thread I may get last post from the 17th or 21st (quite correct but random and not exactly active 
)
If I click on a particular subject, eg chat, I get an up to date list
Weird! Anyone else?