For example, while Facebook Groups does allow commenting by email, if you attach a picture to a reply, it doesn't appear in the group.
Secondly, I don't see many small businesses or even teams within large companies using Facebook Groups to collaborate.
Not sure if that's relevant, since I don't see any way that they start using Posterous for that.
Whatever, I'm just throwing out my opinion that this doesn't seem like a big deal in Dec 2010. Maybe the ability to attach inline pictures will make all the difference, though.
Many businesses also use Yahoo Groups or Google Groups today for email lists. Yet those products are pretty lacking.
We think there's a real need for a modern email list solution and we're confident we can get users from individuals to businesses to bring their lists to Posterous.