I think it's settled for the next decade or so at least.
With 687 million users if each user spent 15 minutes switching to an alternative it would take a combined time investment of about 20,000 years for Facebook to disappear.
Of course they don't have to lose all their users to become irrelevant..
If Facebook's growth stopped today and they began to lose users at a rate of about 90,000 per day it would take 10 years to lose 50% of their users.
There's a class of kids out there who are 12 right now; by the time they're 16 or 18 are you sure they're going to want Facebook accounts? According to researcher danah boyd, these kids will probably have created and disposed of many different social networking accounts by that time.
I'm not suggesting Facebook is unchallengeable just that it will take much longer due to the enormous number of users. The exodus from MySpace took several years with about 1/4th less users. So a 10 year timeframe seems completely reasonable to me when you're talking about almost 700 million users. I just can't see how that happens in 4-6 years as you suggest. Facebook would have to stop growing entirely and begin losing hundreds of thousands of users per day to a competitor.
With 687 million users if each user spent 15 minutes switching to an alternative it would take a combined time investment of about 20,000 years for Facebook to disappear.
Of course they don't have to lose all their users to become irrelevant..
If Facebook's growth stopped today and they began to lose users at a rate of about 90,000 per day it would take 10 years to lose 50% of their users.