Small, non-interruptive popups are great for subscriptions even if people don't enter your signup flow. I think you should keep that on your site regardless of how it's performing vs. signup conversion.
For your social signup folks - adding another step in the flow isn't uncommon or even annoying, if you use the step to convince users of the value of subscribing. After continuing signup with Gmail, the New York Times site shows a clean, targeted message selling the value of a paid subscription, and one click to start the opt-in. For an unpaid ordinary marketing email subscription, you could probably do even less. Just allow the user an easy way to say "no thanks", and that should cover your compliance requirements.
Before logging in with social:
Right after social login:
