And the next one to charge is...
Providing a "free" service is purely a numbers game - if you have big numbers of users (Hotmail, Yahoo, Gmail and so on) then advertisers will pay. If you haven't, they wont. That's it.
It would take a really big player (say BT?) to try and break into that now.
Personally, I would happily pay a few pounds a year not to have adverts, but I don't seem to get that choice.
What I really would like is to be given the 8mps BT promised me - and for which I DO pay - but that's another story...