WIth the exception of Twitter, the TOS of almost all services already prohibited adult content. For example here is the first item in Snapchat's community guidelines.
Has Sanpchat shut down models accounts?. Yes. Is it a majority?. Not even close, there is only one model out of roughly a dozen Snapchats I follow that was shut down and her non-nude tease account was left intact. Snapchat faces far bigger legal issues, with child porn law with teens sending nude snaps to each other.
The big difference with FOSTA is that it ends the protection given to website/service operators by section 230 of the Communications Act of 1934. That Act protected the liability of the website for illegal posts made by users of the site. FOSTA puts an end to that. That’s why people are watching what companies will do. Before FOSTA a model could breach the TOS and it really wouldn’t affect Snapchat legally. If a person Snapped child porn, the user would get arrested/punished, not Snapchat. FOSTA will permit the government to fine/arrest Snapchat operators for content that its users put up that is promoting prostitution and/or trafficking.