Disclaimer: I started working at FB recently. Consider the following model scenario. You are a PM at a discussion board startup in Elbonia. There are too many discussions at every single time, so you personalize the list for each user, showing only discussions she is more likely to interact with (it's a crude indication of user interest, but it's tough to measure it accurately). One day, your brilliant data scientist…
Very high levels of engagement seems to be a negative indicator for social sites. You don't want your users staying up to 2AM having arguments on your platform.