I have a Facebook group about philosophy, and it gets 1000 members. How should I celebrate this, and encourage people to engage and interact?
First, nicely done! Your efforts have lead to over 1000 people joining, reading, and potentially contributing to your group. That's a great achievement for you, but it's also a great achievement for your community, so start out your post by mentioning that!
Given that your group appears to be continuing to grow, in your milestone celebration post you could perhaps mention how the group started and how proud you are of your contributors. Perhaps shout out some of the group's most significant contributors, and thank them for their contributions. Maybe even link to the highest-traffic posts so new users can check out some of your group's finer contributions, and link to older, but interesting, posts that folks may not have seen yet.
Round out the post with a look towards the future, encouraging users to bring others into it, and maybe once you've reached 10,000 members you'll have more stories to share and contributors to thank.