Needn't be an issue.

What I envisage is that it will be probabalistic. If you go to a party, you only get the content that was generally popular amongst that group of people. You have no way of knowing which individuals were interested in that content.

So, if you go to a Metallica concert, you might be exposed to Metallica content. No privacy issue.

That would also take care of the porn aspect. Unless the majority of your friends are into the same kind of porn, you simply won't see it. And if you do see it, it can only be because you have deliberately hung out with a group of people having that as their common interest, in which case you shouldn't be surprised to see it.

Let the numbers of the crowds take care of the filtering and privacy aspects.