You can also ask friends to put their contact details on Facebook and share specific details with a limited list. That's what I do. I also use iCloud to sync contacts on my iPhone automatically. I treat people who lose phone numbers even when it is so easy to sync these days as someone I really don't want around. Seriously, it's stupid not to sync this important information. I call that Modern Technology Darwinism. Only the tech savvy will survive the next 100 years. (I am only half joking.)

Android and iPhone also sync using Twitter and Facebook. If you are not syncing you are just lazy or stupid.

As for staying up to date, only old forms of communication don't change smoothly. If we are connected via Facebook or LinkedIn you can always reach me. If my phone # changes, which is not necessary with number porting now, I can still be reached by Twitter, Facebook or LinkedIn to ask for my #. I see no need for this Knok service.

...this is why i have an android...syncs everyone...always...