My conclusion (and this isn’t a legal opinion so don’t take it as such) is that the Courts may not view social media sites as a public forum. They may even go so far as to say it is government speech.
I will say that the following from the Washington State Attorney General’s facebook page put my mind at ease “The AGO just can’t have swearing, naked photos, political rants, or pormotion or advertisement of sservices or products on this site. This is a place to find and share information related to the work of the AGO, so let’s keep it to that, okay.”
Like that Washington State…especially the tone.