I was looking few days for something like this and look what I found!

with this app you can follow your friend online activity and get a notification to your iPhone when the friends you are following are online , and also when they checked in near you!!

Exactly what i was looking for!!!

You should try it , i love it..

Who's online : Who's Online for iPhone 3GS, iPhone 4, iPhone 4S, iPod touch (3rd generation), iPod touch (4th generation) and iPad on the iTunes App Store

There is a free verso to called Who's Online Lite