At Twitter, we see spamming as a variety of different behaviors that range from insidious to annoying. Posting harmful links to phishing or malware sites, repeatedly posting duplicate tweets, and aggressively following and un-following accounts to attract attention are just a few examples of spam on Twitter. Like it or not, as the system becomes more popular, more and more spammers will try to do their thing. We’re constantly battling against spam to improve the Twitter experience and we're happy to report that it's working.
I have noticed a good number of spam followers are nuked well before I get to check the profile.
I'm seeing a smaller number of blatant spammers, but I'd be much happier if Twitter applied some kind of quality scoring algorithm to root out the slightly more subtle spammers.
Every day I block new followers who have thousands of friends and followers, certainly grown through automation, and mostly low quality tweets - quotes, retweets, and, of course, links to whatever they are selling.
Part of the problem is that almost all Twitter users fail to act aggressively to stamp out these promotional accounts. One third-party tool I use to vet my new followers tells me what others have done. Even for accounts that I consider blatantly promotional, typically at least half follow back. It's very rare to see more than a few percent block an account like that. The balance "ignore" the account, i.e., they don't block or follow back.
If more tweeps got aggressive about blocking, it would give Twitter some data to work with.