It takes a few years, but once you're no longer a growth company and the lay offs start up, the company because a less interesting, more stressful place to work. And the best people stop wanting to work there. (See Yahoo, Aol, etc). What's going to happen now is twitter is going to try and trim down their own projects to siphon as much profit as they can from them, and then try to grow by acquisition instead of innovation...
So what? This is happened to every successful startup in existence. A company can't be a startup forever. What it needs to find is a niche which can continue churning out successful products even when the majority of employees are concentrated in the range of 35-50. This is what Microsoft, Intel and Qualcomm do and they're still largely profitable companies.
