Wappalyzer is a chrome extension which tells you about the front-end and server side technologies used in a website. A while ago when I was using Facebook, wappalyzer said that Facebook uses Vue.js for the news feed. It looks strange to me, because having a mature front-end library like React, developed by Facebook, why would they use Vue.js for a major part of Facebook. I read somewhere that Facebook used Vue.js for some small product, but using it for something like news feed really looks strange to me.

It's a false positive caused by a problem of the virtual dom, similar to ReactJs one. It's not the first time they happens.
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stuff ;)
Atul Sharma
Full Stack Developer | Cloud Native Applications
Seems to be a false positive reported by Wappalyzer due to somewhat similar implementations of virtual Dom.
I have read a similar debate over Reddit few days back.. where some people claims that they have checked compiled js of Facebook and found some code related to Vue.js . If it's true then it's a great news for Vue developers that Facebook considers Vue as good alternate to react, production ready and stable .