I rent the pinned post in many of my facebook groups (list here: https://ultimatevirtualsolutions.com/facebook-support-groups/)

Rental period is one (1) calendar month. Cost is based on the number of members in the group:

1 - 4999: US$50.00
5000 - 14999: US$100.00
15000 - 34999: US$200.00
35000+: US$300.00

The only real restriction is that the post must be on-topic for the group. A post about a plugin would be welcome, obviously, in the plugins group, but also in WordPress Solutions, WordPress For the Non-Technical, and other groups where it would be on topic as well.

Most of these groups are "open" so anyone who finds and visits the group during the rental period will see the pinned post as well, even if they are not a group member.

My WordPress Plugins group was noted in Elegant Themes' list of "The 20 Best WordPress Facebook Groups You Should Join" (#9 in the list). See the complete list here:

https://www.elegantthemes.com/blog/resources/the-20-best-wordpress-facebook-groups-you-should-join

And, for example, the WordPress Plugins group has 475,500+ members meaning it will be US$300.00 for 1 month.

One final point. Additionally, I will be doing Zoom and facebook Lives covering the content of pinned posts in the various groups -- at least one Live for each pinned post -- throughout the coming year (2021). These presentations will be on Zoom and facebook, on my own timeline, in related groups, and in my own support groups, some of which are private and/or paid. This will greatly extend the exposure of pinned posts starting in 2021.

To book a month, send funds to my PayPal account: https://paypal.me/OurHutchLLC (Lora is my partner), or request an invoice. You can use major debit or credit card for payment. You do not need a PayPal account. After payment is made, add the post to the group. I will verify funds have been received and pin the post to the top of the group. Always best to check with me first to see if the group has already been booked (you can still book now to secure the next available month). The number of bookings has been steadily increasing.