You are correct that as a public company with shareholders they are required by law to "derive value" for those shareholders, however on the email front tarring google with the same brush is a little unfair.
This move by facebook, as almost everyone concedes, is about trying to force people to remain their site, creating a very morally questionable form of lock-in, and corrupting the word "email" as their service has almost nothing to do with the word as we have come to accept over the last 30+ years. Google, when they released gmail, while going for ads to display on the webmail, were pretty honest in following the standards of email. I mean it was a "what? really?" moment when they happily provided full free imap capabilities, something that at the time was a service that attracted a hefty premium. To take it to its limits I can set up a gmail account, have it forwarding to a non google email addy, and never look back. Yes the emails pass through google's servers, but if I never log in what use it that to them in either displaying ads for me or for better targeting me through doubleclick etc. as I browse the web.
So while google are far from saints with some of what they do these days, on things like email they follow conventions/standards very well and deserve praise - facebook are not interested in anything but ensuring that you spend as little time on the internet as possible outside their domain (literal and figurative).