With the rise of Twitter, we've also seen the rise of link shorteners (standard URLS take up too many characters). The king of the link shorteners right now is bit.ly, which is the default shortener on Twitter and accounts for more than 75 percent of all short URLs on the service. Every month, bit.ly shortens about 1 billion links. For spammers, that's one huge honey jar.
The flip side of a short link like this one—http://bit.ly/6PwhcP—is that you can't tell by looking at it what website it redirects to. It could be a TechCrunch post, or it could be a spam site. There's no way to tell the difference when you see the link in a Tweet. (Don't worry, it's a TechCrunch post).
The spam problem is getting worse, which is why bit.ly is taking more serious measures to sniff out spam behind its short links. Today it announced it is working with three new services to fight spam and malware: VeriSign’s iDefense, Websense Threatseeker Cloud, and Sophos.