One thing I miss from the golden age of #Facebook, when pretty much everyone was on it and kept their #profile up to date, was that it was easy to find how to message someone. When I wanted to contact someone who I hadn't talk to in a while, I just used Facebook. I didn't had to worry that I had a wrong or outdated email address. I could see their profile to make sure it's the right person, and that they had been active recently so my message wouldn't get lost in the void. It was reassuring.
You could open that web address using any browser to see the person's public profile, which is a good default if you don't have a compatible contacts app. This public profile would list public information that your friend want to share, like maybe their Linkedin profile or work phone number.
But if you do have a compatible contacts app (and ideally, all currently existing contact book apps would support this open standard) you can add that URL in your app to save the profile of your friend. At the start, you only see their public profile. But you can then send a friend request to see their private info, which would include things like their personal email address, personal phone number and home address.
You should have total privacy control of who as access to your personal information. For example, you could hide your phone number by default and people would have to send you a request before seeing it. And would could decide for how long to give it access to: forever or maybe just a couple months? (The default could be something like 2 years.)
Another difficulty that come with decentralization: What if you want to migrate your profile, including all your address book, to a new host because your current host is problematic? It should be easy to migrate, without losing any of your friends or their data.