tedster - 9:01 pm on May 26, 2011 (gmt 0)
Twitter spam accounts are easier to filter out than spammy backlinks in general. It's very hard to create a huge network of spam profiles that are naturally integrated into the larger Twitter community, the way a real account would be. Spam accounts do not engage real accounts very well, and almost never get return engagement from them, except to out them publicly as spambots.
Any kind of large scale analysis would show these patterns quite dramatically, and both Twitter and Google are doing that kind of work.