It certainly won’t hurt us. The other angle is that many people are now considering leaving Facebook that hosting your community may make it more difficult to reach your target audience.
Users may need to reset their passwords (if they never set one in the first place), but there should be no other side effects.
You may want to provide some instructions for your users. Let them know that accounts are based on email addresses, so if they always logged in via FB they should do a password reset to their FB email address and use that to log in.