Your broad definition of a hybrid app might be too big. The entire application running in web browsing is sufficient, but not a prerequisite for a hybrid application.
You can imagine this as a spectrum:
Pure native Facebook Appcelerator PhoneGap HTML5
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PhoneGap applications, for example, such as you mentioned: "applications that transfer WebView to their own application." However, Appcelerator applications are also hybrid applications, but they do not have to contain a web browsing component. They are hybrid because their user interface is native, but parts of their logic work in Javascript.
Facebook is definitely not built in PhoneGap. Creating your own sliding menu in HTML5 without any performance crashes is nearly impossible with current webviews. There are signs that Facebook is not built in Appcelerator. In my experience, Appcelerator applications tend to have huge file sizes due to its titanium libraries. Facebook should be much larger than its current size if it was built in Appcelerator.
Facebook has the resources to create its own applications, so for common reasons it does not require a structure (development speed, ease of coding).
Finally, and perhaps the best reason I would say that Facebook is not built using a (publicly available) structure, is that if it were, this infrastructure would be 1) happily announcing it to the world or 2 ) from Facebook.
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