PlayStation Network will now let you add up to 2000 friends, a significant boost from its original limit of 100. The functionality is already available on Vita and will be ready on PS4 at launch.
While you can add up to 1000 friends on Vita, that functionality won't expand to PS3. The PlayStation official forums (via Joystiq) explain that "if you build a friend list bigger than 100 on PS4/Vita, network servers will automatically filter your friend's list when you play on a PS3 system. Additionally, you will be able to set and lock a pre-determined friends list to be used while you play on the PS3 system if you wish."
Apparently the ability to display all your friends in applications and games is "planned," but obviously some games will be limited to the original 100 friend limit.