When dealing with guesses/estimates, it's a good idea to be a bit cautious and not trust the numbers too much since there are a lot of variables to consider...
Things like:
- How many people have multiple blogs/sites?
- How many people transferred from one to another? (Maybe started n Tumblr then moved to Wordpress.)
- How many are actually active/in use or have at some point been active? (I mean there are plenty of people who register a blog then do nothing with it. Maybe just camp on a particular url since they think it may be popular. Or intend to do something but then don't.)
- How much bleedover is there? (I mean I own both a forum and I have an account on Tumblr for promotion of my forum... And how many are like me?)
That stated, it's difficult to know exactly how many forums or blogs there are since there are free hosted options for both (which are easier to know numbers since they often post milestones). But how many paid-host options are there? (People who own hosting and run a blog or forum? Or both?) Since it's very difficult to accurately measure all of them. (Which could easily inflate forum or blog numbers to a higher level.)
Though I think we should also be somewhat careful since 1 Blog/blogger doesn't necessarily equal 1 forum/forum admin. (There is some crossover to be certain in that both have to promote their site/blog.)
But one forum is very often larger than a single blog might ever get. I mean a good sized forum might have well over 10,000 posts. And may grow at a rate of 30-70 posts a day. And there aren't many bloggers that can really compete with that sort of numbers per day (at least not for very long anyway.)
So in that regard, a forum is more like the blog site's owner (like Tumblr or Wordpress rather) than a blogger. And each member of a forum is more like a blogger. And that kind of changes the numbers a bit.
For interest sake,
InvisionFree reports they have over 2.7 million forums with 40 million users, meanwhile
Zetaboards reports 3.8 million users on 120,000 forums. (Though those numbers haven't been updated in at least 2 years. So it's fairly safe to assume that they've since gone up.)
Though, like mentioned above with Blogs, some caution should be taken here since we don't know if they're reporting individual and unique users or every account that gets registered. (Which is important because some people register on multiple sites...)
But if we go with the number of registered users for forums rather than number of forums, I think things will even out a bit more. (Or possibly even tip the numbers in favour of forums... )