PS The Alexa toolbar add-on gave stats for sites you visit, and there are sometimes web trend reports giving lists of top 50, top 100 sites, by traffic/visitors but many of the statistics will be related to worldwide figures, or not specific to forums, or might include websites like the BBC (where there are so many pages, it may be unclear from any outside inspection what portion is for strictly viewing news or programme information pages, what portion would be viewing tourist-type info - what's on, etc, at county or regional level, weather/traffic and what portion, for you, might be of interest - participation on the Blogs as users can comment, participation on the messageboards, and those who comment on news stories, perhaps.
Other forums will include topics from train spotting, sports (football team sites), all the way through.
I view a few different boards, mostly to do with technical topics, but use NewsNow.co.uk to find subjects which interest me (items on personal / company blogs for example) as there are no blogs or single sites I routinely check daily (no twitter feeds, no facebook pages I check, no personal or news blogs individually).