On a somewhat related note, it's my opinion that Slack could be using private DMs for advertising purposes. My friends and I were skeptical at first after they started getting ads in Instagram/FB and other platforms immediately after we'd mention a product. We figured it was coming from another source. We then started testing it by mentioning products in our private chats that we'd never searched or talked about elsewhere (Fish Food for example, none of us have pet fish). Lo and behold we'd start getting ads for them.