Family Guy uses celebrity jokes and the like, but only as quick cutaways usually. And there's also actually a joke there. South Park basically expects you to roll on the floor laughing because they have a crude caricature of Paris Hilton or whatever onscreen, aka exactly what Friedberg and Seltzer do.
If anything, Family Guy is a lot more like Selzter/Friedberg because it doesn't use cultural references to advance the plot in any way. It uses them for comic relief.
And that's ok. That works for a TV show. It makes a horrible movie series, but TV fits it.
