Not sure if this helps, but …
You can build audiences out of people who use the native polling/survey widget in MailChimp…
Then export the list of that audience segment into FB as a custom audience.
We did that to make sure we only sent requests for product reviews to happy customers at my last gig … tripled our monthly review submission in a month, and automated it to use as a LAL in FB Ads …
Might not help with prospecting directly through FB Ads … but you could use the Lead Ads to build your Mailchimp list … survey/poll them there … then export the desirable audience segment back into FB.
Just ideas.