Is the use of Facebook's "pixel" feature for marketing products, which allows for identifying users' inclinations and interests without their knowledge, considered a form of espionage forbidden by Islamic law?

There is no objection to utilizing the pixel feature in marketing, because users' data is visible to website owners, and they offer their services to utilize this data in advertisements. Therefore, using these websites constitutes an implicit acceptance of their access to data and monitoring of activities. The prohibited spying is searching for people's hidden faults, as Al-Baghawi mentioned in his tafsir.