So it seems that the experiment with the chat room did not work out. See also the summary here. — Martin Sleziak 1 hour ago

Oh that's a pity. And wow that's a long time you've tried to keep it up! I suspect a more efficient (even if more haphazard) method would be to simply post the bounty-expired question in a dedicated chat-room for serious mathematical discussion or some specific topic. The problem with that is that there aren't that many such chat-rooms and most of them are empty. It seems chat is where users go to just have fun and usually they would rather get internet points for their mathematical contributions than engage in back-and-forth discussions. — user21820 46 mins ago

@user21820 Well, it is possible that a user who offered bounty is no longer interested in the question. See also Peter Franek's comment on this. — Martin Sleziak 43 mins ago

Oh I was referring to the level of activity in that room, not whether it is useful. Even if there are many people who are interested in answers to questions with expired bounties, there simply aren't people who are interested in looking at those questions, not to say interested enough to seek out such a chat-room. — user21820 38 mins ago