I think as others have mentioned, spam is a big problem, but the more fundamental problem with this idea is supply/demand. For a sufficiently large system, the

I think as others have mentioned, spam is a big problem, but the more fundamental problem with this idea is supply/demand. For a sufficiently large system, the number of people that are interested in talking to person X about topic Y isn't likely to be well matched with the number of people person X is interested in talking about topic Y with. Dating sites, while not what you want, are very illustrative. On open messaging platforms like OkCupid, in which anyone can message anyone, women are barraged with messages from men that want to talk about sex. How do you filter them out? If you do manage to filter them out, how do you surface a reasonable number of good conversation matches? The only solution I've seen in this thread is offering opt-out. But all that's doing is offering people a way to not use a product, it's not fixing the problem. Even relatively well funded and mature dating sites solve this by either offering finicky filter options to users, or really blunt filters (women have to initiate the conversation).