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