I don't know. Massive UI changes, a growing creepiness factor and privacy scandals may have caused a tiny number of people, myself included, to leave but I think you underestimate people's ability to suffer through things. To prompt a mass exodus the inconvenience and annoyance caused by ads would have to outweight the huge benefit of social connectivity with a large number of people. There would have to be a better alternative (G+ ?) and an easy way to migrate without losing all your contacts before most users would even consider leaving imho.
I think Facebook is more entrenched than myspace ever was and that'll make it more resilient. Now everyone is on Facebook not just teenagers, e.g. my 50yr+ aunts and uncles who would have been PC-phobic before. That'll make it harder to leave because of the circles of contacts built up over time.
They'll find an innovative way to monetise users obliquely. I'm betting on search.