I wouldn't be surprised if people think that offering only FB login means you're trying to exploit on FB specifically, but if you have self-login as well it means you're not trying to force them into it.
There's no incentive for me, as a college derp, to turn it off - after all, it's not impacting my experience on Facebook. Sure, everyone else's feeds are annoying, but it's like a mix of the prisoner's dilemma and the tragedy of the commons.
Not to mention, you can hide truly annoying feeds - but again, most people don't care.
My solution is no longer visiting Facebook. I keep my account because it's still the least inconvenient way to plan events.
Disclaimer: FB engineer who uses Spotify all day every day.