I’ve always found that most tools focus on tracking public activity, like posts, comments, and likes—nothing really lets you see messaging behavior in real time. For public stuff, try Social Searcher or similar social monitoring tools; they can notify you when someone interacts publicly. As for browser extensions, there aren’t many reliable ones, since Facebook limits what can be tracked. What worked for me was setting up notifications for their public posts and using keyword alerts. Let me know if you want a walkthrough on setting those up!