FB is an advertising company, like Google but with a different business model to drive it. In this case, FB takes the discovery of the user's interest in grilling and uses it to send ads into the user's "news" feed (or more likely, the sidebar) for grilling-related stuff, or sells what it knows about the user to grilling-related companies etc. This is either innocuous, annoying, or worse depending on FB's mood that week for privacy-related policy.