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.