Recently published research by Nielsen/NetRatings suggest that over half the UK internet population visits "online member communities" in any given month. Friends Reunited comes top of the list, followed by Blogger, Bebo, MySpace and Genes Reunited.
I reckon the list doesn’t give as clear a picture as it could because many online communities are likely to fall outside Nielsen’s definition of "member communities" – for example the BBC or some of the football club websites. The research also probably misses sites like gumtree which have, so far, fallen beneath the radar of most studies like this but which are hugely popular. Still, it might come in useful if you’re trying to sell online community to client to be able to tell them that almost 1.8 billion community content pages were served up in the UK each month – that’s 57 million a day (equal to one per member of population!).