If anything, the preposition would depend on exactly what you were doing. In both your examples, I would use 'on'. Things are hosted on a site. "Can you send me a link to your Facebook post?" Links point to something. "You see much more overt hostility in Twitter posts than you do in real conversations". The hostility is contained in the words of the post. It's a little dated to refer to a Facebook wall now, but people used to refer to "posting something to someone's wall." That's an analogy to pinning something to a real wall. "I usually watch Youtube in the Youtube app". You do things in an app.