I think the difference should be whether they are a dumb pipe, or whether they exercise editorial control and/or promote some content over others.

If you are truly a dumb pipe, that just transmits whatever the users post, then you shouldn't be liable for what goes over your wires. Like the phone company.

As soon as you start acting as an editor: amplifying some content and downplaying (or removing) other content, re-ordering it, ranking it, and so on, then you are placing your name on the content and in a sense should share liability around it.

Companies should have to deliberately decide who they are going to be: are they just wires like the phone company, or are they a newspaper's letters-to-the-editor department? They shouldn't be able to act like one, but have the liability of the other.