I'd love it if Facebook did this. But Facebook don't allow you to sync your FB friends' contact details onto your phone --- it's actually against their TOS for you to automatically extract your friends' contact details from facebook.com.

Would I use a standalone app that did this? Chicken and egg problem: you've got to make your app useful to me even if none of my friends use it.

If a particular friend also uses your app, can you mark their contact details as being guaranteed-up-to-date?

If a particular friend doesn't use your app, can you remember when I entered their details and keep track of how stale they are? If I text them and the text doesn't get through, can you highlight their number as possibly not working any more?