Cool probably doesn't matter nearly as much as it used to given Facebook's user base size. As long as they have eyeballs, they'll have a revenue stream. Their willingness to pay what appeared to be outrageous sums for competitors users reflects that. But now that they've got the lions share of the social network using world, it's doubtful that they'd need to do that again in a big way unless they see something getting serious traction with people who are not already Facebook users.
You say least stable but let's look at Yahoo as an example: they've been dead for over a decade and lots of people still use them. Massive data breaches, people still use them etc. etc. Imagine how many people will keep using Facebook if all they manage to do is be a less bad version of Yahoo. People are creatures of habit and once they develop a habit, especially one that doesn't cost $$$ upfront, it takes a herculean effort to get most of them to change their behavior. Once people are hooked, many of them seem to be willing to be abused nearly forever.
You say least stable but let's look at Yahoo as an example: they've been dead for over a decade and lots of people still use them. Massive data breaches, people still use them etc. etc. Imagine how many people will keep using Facebook if all they manage to do is be a less bad version of Yahoo. People are creatures of habit and once they develop a habit, especially one that doesn't cost $$$ upfront, it takes a herculean effort to get most of them to change their behavior. Once people are hooked, many of them seem to be willing to be abused nearly forever.