Yes, that does make it harder. But a few things make it easier too: 1. Twitter can see all messages from the sender -- if the stream contains a lot of suspect

Yes, that does make it harder. But a few things make it easier too: 1. Twitter can see all messages from the sender -- if the stream contains a lot of suspect material and was created recently (for example), that makes any individual Tweet more suspect. No individual mail client can do this. 2. A 'false positive' in Twitter has a lower cost than in email. People ignore tweets all the time, so hiding the wrong thing sometimes won't matter so much. 3. Twitter is a more constrained platform, so the problem domain is presumably more limited. I don't mean to say it's easy or even "not very hard" to pull this off -- just that it is either well within Twitter's capabilities, or soon could be.