Only thing you likly can't do is some of the social features across PS3 to PS4 like the chatting. And since the PS4 can't play PS3 games you won't be able to play with each other any way. Keep in mind though you can still have it active on PS3 so if you are on the PS4 playing online you can easile turn your PS3 when ever to play with thoes still on the PS3.
Ps3 players can't see Ps4players online or what they are playing!! my ps3 friends say i'm showing up as not having logged in since launch day. I'm logged in playing resogun & BF4 on ps4 and can message with ps3 friends but they can't see that i'm online or what i'm playing or how I am messaging them until I explain it for 400$ why screw up the friendlist? this is stupid sony needs to patch in this feature it wasn't smart to overlook this! To see that a player is online and what they are playing is an important part of the gaming 'experience' they must not understand that I would expect something like this out of micro$oft not sony please fix it with next update so ps3 users can see ps4 users online!