@caiqueaccpasquale Hello, difficult to say for sure without taking a closer look so it might be worth raising a Support ticket so we can investigate this further.
I'm guessing you are using the Peole-Based Facebook destination? This could be something to do with the setup of the destination itself but the number of IDs recognized in AAM will also depend on the ID synchronization you have done between your own 1st party IDs and the corresponding hashed email addresses
See this documentation
People-Based Destinations bring the notion of Shareable Audiences to Audience Manager. This metric helps you understand how many of the hashed email addresses Audience Manager can share with the destination platform.
.. again, a Support ticket would be best for a proper investigation