Part of the reason not to let users pay is it could decrease the value of the ads they do show. The 8% or so of people who would pay for twitter are probably al

Part of the reason not to let users pay is it could decrease the value of the ads they do show. The 8% or so of people who would pay for twitter are probably also the more influential, heavy twitter users. Thus, the 92% of people getting ads might be the lower-value users. That means their current ad revenue might drop drastically. But they could still make a few hundred million dollars from that 8%...