Turns out it was Joomla's fault, they accidentally reverted some routing changes in 3.8.4. Updating to Joomla 3.8.5 has fixed the issue and my users can use social login again:
https://www.joomla.org/announcements/re … lease.html
After updating Joomla to 3.8.4, Facebook login is no longer working. When it tries to hit this url after clicking the Facebook button:
it returns: ERR_INVALID_REDIRECT
Any help would be much appreciated. Thanks!