How can I use the collected Facebook contact database?

Question

Facebook

Our site has an authorization button via facebook and via email. After registration, users leave applications for services. But here's the problem - how to contact these users who left the order. If everything is clear with email - I send them a letter and that's it, then what to do with pre-orders from Facebook?
1) I tried to write a personal message to everyone on Facebook - Facebook has a limit of N messages per day, but at first it blocked my account altogether for "spam"
2) Is it possible to send out via API? Is it possible to make notifications in the user's feed from your application? (In the VK API, there was such an opportunity and it made sense to accumulate a user base in VK with access to them)
3) Tell me what other ways to access my specific Facebook contact database are there?
Thanks

Answer 1

And if you show ads on FB to them? I'm not sure that there is such an opportunity, but you can dig in the retargeting settings.