Twitter shares fell 4.5% today following a disappointing fourth-quarter earnings announcement on Wednesday. Wall Street is valuing the company at a little more than $6 billion, excluding its cash reserves. That’s five times less than it was worth a year ago. That’s why Twitter has drawn some interest from private equity firms and others who think it may be undervalued and worth buying.

But despite its drop in value in recent months, Twitter is far from cheap, so don’t expect M&A activity anytime soon.  The pool of potential buyers is tiny, and Twitter’s stock-based compensation for employees is among the many hurdles.