I honest-to-god don't believe this forum is being "targeted" by anyone in particuler.
I think it's just the "Evolution of a Forum", where spammers find your adress.. Hell, I used to run a phpBB forum with a little over 15 active members, and that in itself was getting more spam than this forum has ever been.
Like Amarth said, though, it aint going to work for ever, however, for now, I can't see why not... Commercial solutions especially are almost guarenteed to have less spam due to their smaller user base.
That's not to say SMF is as bad as phpBB... I still think SMF is a better option than IPB, atleast for the moment. It's free and has no where near the same number of spam bots as phpBB...