IMHO, because they do perform curation, both algorithmic and manual, they should be considered publishers.
The problem is that being a publisher brings greater legal liability for the content that they publish; whereas as carrier/platform can wash their hands of the data that they transmit and claim that they have no part of it.
I'm not convinced of that. Through technical and billing means, phones encourage one-on-one conversations while discouraging conversations with multiple participants. By disincentivizing certain kinds of conversations, they disincentivize certain kinds of content. It's hard to say exactly what sort of impact this may have on society, but I doubt it doesn't have any.
This may be a far cry from Facebook's deliberate algorithmic tweaking to manipulate the emotions of their users, but I think it's interesting to consider in it's own right.