How many nieces and nephews do this..?

If they're close to you, they'll keep in touch. But bear in mind teenagers are often distracted and forgetful. Make sure they can contact you via email and facebook - I contact my cousins, and they contact their aunt (my mum) that way. Also, it wouldn't hurt to ask them to keep in touch - just occasionally, let you know their news and things like that, otherwise they might not be quite sure what to do and how to approach the situation.