Usually these are "acqui-hires," acquisitions whose only purpose is to employ the start-up's staff...
Because the staff have been hired to be part of Google or Facebook or AOL, they are now part of that company's grander scheme, a scheme that rarely includes whatever website or service the start-up was originally developing.
...there is something fury-inducing and, I would say, morally wrong in start-ups persuading thousands of people to devote their time and energy to using a service that is summarily erased once the owners have been paid off. Yes, the owners may have worked hard, but without those users' efforts they would not have their payday. They may well have had an incredible journey but, time and again, ordinary people are being led up the garden path.
hat tip Jeff Sisson