This isn't even a question to me: it's obvious it was to take out competition in social networking. Facebook has network effects but their organic attempts at starting new apps constantly fail. Acquiring the ones that take off is the only avenue left.

There's no other reason to spend $19B for a company with little to no revenue (and like 3 employees?).

There is a bit of chicken-and-egg here because as a former tech co-founder, i'd argue that much of the risk of entrepreneurship was ameliorated knowing a larger player could acquire you if you do well. I wonder whether -- if FB/etc were not allowed to acquire budding successful startups -- whether

0. Founders would take the risk

1. Early engineers would take the risk

2. Angels/VCs would invest as much in them

In which case you get fewer/none Instagrams/WhatsApps, because the risk is so much higher if your outcomes are limited to IPO/cashflow-business/lifestyle