IANAL but I think you would be liable. Newspapers get sued all the time for printing things that aren't true. The reasoning is that you have conscious control over what you say.
So in your example, a Tumblr blogger could get sued, but Tumblr would not be. The provision in the Communications Decency Act was intended to shield internet service providers like AOL. Before the Act passed, let's say AOL had reviewers looking over comments and removing some of them. Someone could then turn around and sue AOL by saying, "Hey look, your reviewers let this comment stand and it's defamatory!" Whereas if AOL had taken a more hands off stance, they might have have better off legally. Congress put in the provision to encourage internet providers to police their content without fear of legal repercussions. It's sort of like how Good Samaritan laws protect people who try to help accident victims.
You can get sued for anything. Whether or not the lawsuit has merit or not is another matter. I think that's the key here. Would you be liable give my hypothetical scenario?
So in your example, a Tumblr blogger could get sued, but Tumblr would not be. The provision in the Communications Decency Act was intended to shield internet service providers like AOL. Before the Act passed, let's say AOL had reviewers looking over comments and removing some of them. Someone could then turn around and sue AOL by saying, "Hey look, your reviewers let this comment stand and it's defamatory!" Whereas if AOL had taken a more hands off stance, they might have have better off legally. Congress put in the provision to encourage internet providers to police their content without fear of legal repercussions. It's sort of like how Good Samaritan laws protect people who try to help accident victims.