Wait for fan meet and greet day, in the lounges with the lot trait up and coming hotspot trait. Famous sims that show up at the up and coming hot spot during the event are more inclined to accept socials from fans during the event. Usually start out with the lower level celeberties, the more celeberties you know the more likely you will be able to be noticed by the top level celeberties.
Edit: Try to avoid asking the global superstars unless you know at least 2 4 stars or or 1 3 or 4 stars the other ones are only the low lifes but they can get you access to the 2, 3 star celeberties always ask for autographs and selfies from the high levels, they are more apt to accept introduction from a stranger that is a fan than a stranger that is not a fan.
Edit: I almost forgot the only time you really want to attempt social with a 2 star or 3 star is if the two or 3 star is a child of the higher celeberties, if your a friend of the chidren you already got a foot in the door.
