Yes, but you haven't really address 'why' they would do it.

> In this case it seems like the goal of the website is to become a starting point for a person's browsing/searching experience, facebook and google want the same thing.

This is your only 'why', but it's very weak. Every company wants to be the 'starting point for a persons browsing experience', but Facebook doesn't provide that, and neither does Greplin. Google does of course, but Greplin doesn't.

Your point reduces to "companies which consume APIs might compete with the companies which create them, and so might be shut down". Of course, your statement is true, but it's valueless.