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?).

Starting new apps aren’t the only initiatives that fail. Whatsapp was the target to meet or beat for every single speed and reliability metric during the development cycle of Messenger 3.0 (2013).

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

Maybe I'm going out on a limb here, but I think anti-trust in the age of tech and free services needs to be tweaked a little.

Sure, Facebook can buy any company they like, but if it presents a danger to its core business like Instagram did, then the sale should not be allowed.

Maybe Instagram wouldn't have survived without being acquired by Facebook, but let the market decide.

This is why in the EU it is anti-competitive behavior that is targeted, not anti-trust.

Actually the US and the EU both target anticompetitive behavior. The US law is older and so is call antitrust for historical reasons, not because of a legal difference.