Hi Tiago
Thanks for your question.
Facebook Posts are not available through keyword search rules.
You actually need to configure a Facebook page rule. Here are the steps to setup such a rule (after you added your acquisition token)
1. Go to Search Setup
2. Create a new search topic or select an existing one
3. Create a new search rule and select Facebook page rule (the entry below Keyword search rule)
4. Search for the Facebook page you want to add with the exact Facebook url of the page
5. Select the page to add, click apply and then save your search topic.
Here's the relevant part in our documentation: docs.microsoft.com/.../add-rules-search-topic
Please let us know if you have any additional questions
Best regards
Phil