These remarks don't really address the points I was making.
Firstly, if people want to use it, and pay for it, even small charges at mass scale pay off. Something like twitter doesn't cost a lot to run in particular at unit cost.
If they don't, GOTO 10: "the Internet is hamstrung by overreliance on advertising as a revenue model"
I'd suspect that "loads," in fact the majority, of twitter users would not pay anything out of their own pocket to use it.