mike2017 Is there any way to send a digest of emails every week?
There is no way of sending digests yet, not even using Mailgun as @ExZiByte proposes because Mailgun knows nothing about your forum and active threads.
Is it possible to sign in using google or linkedin account?
Yes, I think there are extensions for that. If not, they are quite easy to create.
how active is development, like new features, releases?
The core team has seen a recently renewed boost of activity again. They are now working on segments of the overhaul needed to release a stable version. You could however already use Flarum in production, many people already installed it for that purpose.
