The difference (well, one of them) is that the networks employ those who create the content and those are the people whose words are subject to lawsuits. Social media platforms are not employing those who post. Calling social sites "publishers" is one hell of a stretch, unless someone they employ or pay posts something in their name.
No it's not... They are choosing what can be published on their site even if it's not illegal. Whither or not anyone is paid is irrelevant as far as I'm aware.
Here is what the
legal dictionary Says:
PUBLISHER. One who does by himself or his agents make a thing publicly known; one engaged in thecirculation of books, pamphlets, and other papers.
2. The publisher of a libel is responsible as if he were the author of it, and it is immaterial whether hehas any knowledge of its contents or not; 9 Co. 59; Hawk. P. C. c. 73, Sec. 10; 4 Mason, 115; and it isno justification to him that the name of the author accompanies the libel. 10 John, 447; 2 Moo. & R.312.
3. When the publication is made by writing or printing, if the matter be libelous, the publisher may beindicted for a misdemeanor, provided it was made by his direction or consent, but if he was the ownerof a newspaper merely, and the publication was made by his servants or agents, without any consentor knowledge on his part, he will not be liable to a criminal prosecution. In either case he will be liableto an action for damages sustained by the party aggrieved. 7 John. 260.
4. In order to render the publisher amenable to the law, the publication must be maliciously made,but malice will be presumed if the matter be libelous. This presumption, however, will be rebutted, if thepublication be made for some lawful purpose, as, drawing up a bill of indictment, in which the libelouswords are embodied, for the purpose of prosecuting the libeler; or if it evidently appear the publisherdid not, at the time of publication, know that the matter was libelous as, when a person reads a libelpresence of others, without beforehand knowing it to be such. 9 Co. 59. See Libel; Libeler; Publication.