"Is whoring your users from advertiser to advertiser really that profitable?"
Short answer, yes.
Bear in mind that various traditional media, with far more real-world costs (newsprint, radio, TV) can make good money purely on selling adverts. Hence why "free" papers continue to be profitable.
Facebook is a media company where other people generate the content. Much like Google and the various other social media platforms. It's just much much MUCH more personally targeted, both data gathering and in advertising.
The part that bothers me most is less the shilling of goods and services, it's the influence and manipulation that FB et al are being paid directly to do. Political advertising is obviously not illegal, but is influential when you can target specific demographics with just the right sort of article/ad.
FB and the Tories have kept a bit of a lid on it, but the 2015 election win is widely held to have been due to micro targeted advertising campaigns through FB aimed at swing voters in tight races. There's some suggestion that the negative campaigning worked even better, persuading Labour voters to switch to SDP/Greens/UKIP in races the cons couldn't win.
Considering how effective gerrymandering has been in the UK and USA for breaking representative democracy, and the resistance to any form of change to the systems, this will just allow an even more effective form of forging consent and ensuring people vote the right way, even if for the wrong reasons.
No idea how to deal with it either. Obviously banning political advertising is Not Cool, since that can pretty much be used to cudgel any dissident opinion, and requiring warnings like "this utterly baseless piece of propaganda was brought to you by $Party" is going to be dodged pretty quickly, since there are plenty of think-tanks etc that can be used to front someone else's agenda.
So since FB can probably now sell you the ability to win an election, I'm pretty sure they've got their money+power printing machine running quite nicely thanks.
I'm sure there's a modern equivalent to "Never pick a fight with someone who buys ink by the barrel", it'll be interesting to see what happens if some party pisses off Zuck enough for him to break them.