Very many sites (including the ones I spend most time on), have content which is mainly or entirely generated by the users of the site, not the owners. Does this user-generated content have monetary value? Or any other kind of value?
Facebook, for example, is in a very different market, financially, to both h2g2 and Stack Exchange. And Wikimedia isn’t interested in financial considerations at all.
Incidentally, over on h2g2, Mr606 points out that the financial figures Nick mentions for Twitter are lower than reality: they have been massaged so that Twitter can declare its profits in Ireland instead of in the UK.