They've been around forever. "Everyone has it but no one is on it" was always the description. One good application is finding someone you USED to work with (if they have email notifications turned on, of course). And hearing where your friends are working now. As for revenue: job listings. That's it.
This is literally the only thing I care to use it for so of course it’s nearly impossible to use it this way.
Would love to log in once a year and see a simple list of “person X is now working at company Y”