Often the acquiring company tries their damndest to keep them on. (At least with these "let's acquihire the small startup" scenarios - mergers of truly large, established companies do often see a "merge and then fire half the staff" pattern.) They do things like paying in stock with a vesting schedule to encourage you to stick around. The culture shift is often too much though, and people happy at a small start-up are very much not as one tiny bit of a large conglomerate.

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