Facebook shares plunged 5% this morning after Mark Zuckerberg announced changes to his platform's newsfeed that will prioritize engagement over time spent.

Why this matters: Wall Street analysts have grown accustomed to measuring success of tech platforms by how much time users spend on them. Facebook's user base is becoming more and more saturated and now that it expects people to spend less time, it has made investors nervous.

But, but, but: If anyone can manage this challenge, it's Facebook, which defined the concept of a "monthly active user" as a metric for success early on — which has since been adopted by Wall Street and other platforms. 

Go deeper: The full story on Facebook choosing friends over publishers.