For special days, I try to let them call me first, but if I'm tired of waiting for them to call, I'll call. I see my youngest almost every day and I feel like something's missing if I dont see or hear from him. He knows I'm this way about him.
My oldest is more of a free spirit and is more likely to kind of drift in and out of contact. Sometimes we'll text or talk a couple of times a day, then we wont be in touch for a couple of weeks at a time. I try to let him set the tone, but if I think it's been too long I call him or his wife.
So basically, I guess I exercise my mom rights and call them when I want. (We do a lot of facebook too)