"I also disagree that relying on advertising revenue is such a bad risk for Facebook: so does Television and their business model has been succesful for a loong time. "

It's not a problem to rely on advertisements as a main source of revenue, but Televisions have commercial breaks. What Facebook is afraid of is that most of their users won't even use the website at all, and only the mobile app. Or to continue the Television metaphor, it's like having all the TV watchers switch from TV, to TVs without commercial breaks. poof goes the revenue.