Okay so you have group A who has ads they want to put in front of potential customers and group B who has digital billboards and wants to sell them. Group A doesn't know where to advertise and can't identify customers, and Group B knows about their users but doesn't know what kinds of buyers might be interested in them.

I genuinely don't know how this doesn't precipitate a market-maker who can match up the two groups and who naturally becomes huge because the best matchmakers will be the ones with the largest populations of both groups. The bigger they are the more and better matches that can exist and everyone (theoretically at least) gets better prices because buyers and sellers are in competition in a larger pool.