I've been hearing that about Twitter for years now, and I'm still waiting.
Twitter might still not have a viable business model, but they can experiment (now that they have a huge user base and data) and try to find one. While its not easy and obvious at times to find a viable business model, just reaching the sheer distribution scale as Twitter's is a huge hurdle in itself that very few incumbents will ever reach.
The infrastructure of Facebook and YouTube and Twitter could be replicated by a small team of smart developers in a matter of months, probably costing only a few million dollars. The only thing they have going for them is network effects, which are less powerful than we think when there is a viable competitor (e.g. MySpace being overtaken by Facebook).