Personally, I really didn't mind the advertisement banners when they
were here. They weren't obstructive (i.e. the bottoms of posts and such; they weren't huge), and they were actually about interesting things. I would much prefer they came back than Nabble went out.
As far as Nabble getting more users goes, I think it's on its way. However, to speed this along, I might recommend getting some popular sites out there to use Nabble forums (maybe easier said than done). I still think adding the friend feature will help Nabble a lot in this regard, as well, adding a more community feel to the forums. That, and the ranks for the number of posts (i.e. if you get a higher rank for having posted a larger number of posts, it's an incentive to post more—I know it works with me, anyway, on the lemmasoft forums: see
http://lemmasoft.renai.us/forums/). The ranks actually have names, though; they're not stars or numbers (i.e. Newbie, regular, veteran, etc.); being able to customize these names per forum would be a great way to enhance the user experience, I think (of course, posting a million posts in Nabble Support shouldn't affect your rank in Fantasy Writing, and so perhaps there should be a Nabble-wide rank as well). I highly recommend looking at how they do it in the forums I linked to. Just look at individual posts and you'll see the info off to the side about the person.