I have also considered implementing Hubzilla style DMs in postActiv, but one factor to consider is that in that model the admin is not an adversary, so that could make servers into a juicier target for implantation (especially the ones with thousands of users) and put more responsibility/risk onto the admin. So I still think it's better to use separate more specialized apps for private communications. It also guards against the Lavabit scenario where someone shows up and demands server keys.