They’re not even selling it, they’re just giving it away due to incompetence.
They added the pixel to track their ad click through rate (and to automatically optimize the targeting based on people who click through).
The pixel sends off the URL of the current page when a user visits. The search form put the GPA you entered to search for in the URL, so it gets sent off as part of the URL.
There’s no way Facebook even realized this or utilized the data in any way, it just happens to be in the URL by mistake and they get millions of URLs sent to themselves every second, no way do they actually bother to sit and analyze what’s in them.