I think it's a combination of lots of things... coming here after first finding tools, lots of users signing up after coming into the forum from search engine queries, lots of users signing up because someone told them to, etc. Not really any one thing. It's gotten big enough that Google has an AdSense rep hanging out in the AdSense forum and Yahoo has a Publisher Network/Overture rep in the Yahoo Publisher Network forum as well (I didn't ask them to come).
I know I did my part. I don't even remember how I ended up here in the first place... I think when doing some kind of adsense inquiry in a search engine I stumbled on the adsense forum. This was before I had adsense or any sites... Actually kind of got my started in the whole thing to be honest.