I think a useful addition would be the option to hide a conversation, such that it does not appear in any of the listings. Some things I just don't care about, and I'm quite sure that others feel the same. This would be targeted at threads, not users, and it would override chronology or activity. I suppose one would have to think about some way to undo it, perhaps a special option that lists the hidden conversations. Maybe it would be sufficient to hide them in the For Me tab but not in the All tab, dunno.