Facebook says:

Custom audiences make it easy to target specific customers or prospects at scale. It allows you to match your marketing list against Facebook users in a secure and privacy safe way. Email match rates are typically greater than 60%.” 

https://en-gb.facebook.com/business/a/online-sales/custom-audiences

One example of how this could be used to reach an audience is a campaign to get members to take part in the annual survey.

Let’s say you are an organisation with 10,000 members and you want them all to take part in the annual questionnaire on working in the industry.

Your first round of email open rates are 45%, with an 90% conversion rate (4,050 members) a fair bit higher than normal but the members appreciate being heard so are much more willing to engage with the email and complete the questionnaire.

The follow up email to the members that didn’t open the first email (5,950 members) gets a 20% open rate and a 60% conversion rate (714 members).

That leaves just over half of the members still to fill out the questionnaire. Obviously the website will provide a platform for engaging some members also, and you would hope to get around 2,000 to 3,000 more members to answer the questionnaire via the website.

This leaves around 2,500 more members that could potentially still be reached.

You could always try one last email campaign to the remaining responders at this point and given peoples general propensity to leave things until the last minute you could potentially expect about the same level of engagement as the second campaign so that would still leave around 2,000 members, or 20%.

Assuming that you have at least a semi-active social media presence on both Facebook and Twitter, you could look to explore their custom audience advertising capabilities.