Personal thank you's in email & DM. The personal touch goes a long way.

Also - you could have a batch of custom swag made up. Send it to them with a note explaining the exclusivity.

For instance - thousands of us have the standard grey Mailchimp T-shirt. What if they sent their top 1,000 users that shirt in red with a note explaining the "secret" that only the top users get the red shirt. It's relatively private but the user gets to feel like a rockstar.