It seems that there is a trend of advertising one's crappy service by following people on Twitter. I do a lot of "block and report for spam" on these people; lately, it seems that the strong majority of my new followers are spammers.
Since I opened my Twitter account about a year ago, I've received 243 notices of new followers, yet I have 104 followers. Now obviously there are some people who've simply unfollowed me, but I do a lot of spammer blocking, and I'm confident the vast majority of the 139 no-longer-followers are spammers. Those numbers suggest that spam follows are potentially 57% of all follows; my guess is that the true number is certainly over half.
I don't see an easy way to see a log of my blocks and/or a log of new followers, but I'd be curious to see what the trend over time is.
Anyway, I encourage you to adopt a similar policy of blocking and reporting new followers that have no discernable reason to be interested in you (unless they are individuals with no apparent agenda).