@Hamiller to clarify, the end-user would be unaware of index feeds. They would not be able to choose which feed on which to post a msg. It would all work in the background. The only purpose of index feeds is to allow friends to say "give me all contact messages for Alice, but not the other messages", so that they can replicate less stuff, e.g. leaving out votes which take a lot of space. This means in practice you could browse someone's (say a stranger's) profile but only to see their name, picture, biography text, and followers, not the actual posts. Or maybe you could choose to see the last 100 posts, but not see their votes. That kind of thing. The purpose is simply to allow choosing which content types to replicate.