Are there really different rules on FB? As in something can be vulgar in rl but not on fb?
Ask me in a few months, I'll be doing a MOOC on "cultural norms" on social media as a module....I think..
But generally yes, new forms of communication do tend to take on their own norms and "social no-nos" as they become established. Doesn't tend to be wholly disconnected from real world social norms, but there is flex and context given the differences of medum.
So... you post a pic of pressies under tree, with mainly family/close friends in mind as the "recipients", but all your other (more tenuous/less close) "firends" see it too, cos they have access to your content. That does not translate as being somebody in RL who would trapise around to all aquantences houses to waggle pressie pile at them while preening.
Intent matters, and intent is much harder to communicate/interpet in social media than face to face.
Pretty much, as far as interpeting motivation for what was posted, I think you have to mostly bear in mind what you know about the person. If showing off is not something you associate with them in real life, then that is unlikely to be the best explanation for the pic they posted on facebook. If showing off is just who they are all the time to the point that it annoys you, then for everybodies sake probably best to unfriend in RL and on FB.