I don't see this argument as very strong. Facebook will stagnate only if the company loses it's enormous drive to dominate social and give people what they want.

Facebook is essentially a communication tool. There are two things in my mind that could become the "next Facebook". First is that centralized social networks fall out of favor and people use multiple ones to handle family separately from friends, etc. Second would be that a simpler, more effective alternative comes up. I see Twitter as that. Communication is more direct, simpler and much better tied in across all kinds of devices. Plus there is less stuff (groups, events, pages, games, privacy controls, public/private profiles, etc).

I really like Moots comment (guy who runs 4chan.org) that Facebook is like "training wheels for the internet". It's a secluded, safe garden for the masses. Yahoo is similar, as it's a directory for the internet.