Twitter is quite frankly worse. And I don't know if I am personally acquainted with even a single person who actively uses LinkedIn. I wonder if the latter is an SV thing?
I don't think LinkedIn use is limited to SV, but the qualifier "actively" makes it unclear what you are trying to say. What is "active" in this context?
Marketing it is. I actually thought about this idea for a long time, and did a design based off of the phonebook in iOS 11.
But marketing this application is nearly impossible, because it doesn't have any extra features. It just has phone numbers [contact details], and that doesn't excite users to spend time getting it set up.
So I concluded that this is something that has to be done from the OS vendor side. We need an extension to the CardDAV protocol, and we need vendors to implement it.
A great many people that I communicate with stopped using email regularly ages ago. For younger folks the means of communication are messenger apps and social platforms.