According to Walter Isaacson, who recently spoke to Washington Post CEO Donald Graham, Mark Zuckerberg was among the few people that Steve Jobs held great respect for:
"I once asked Jobs who [he] admired in the Valley"... "Mark's was the first name on his lips."
As you probably remember, Apple once tried to offer a social service itself, in the form of Ping. The service had Facebook integration, but apparently Steve and company didn't ask for Mark's permission. As a result, Facebook cut Ping's access and Apple's service turned out to be a failure (for that, or some other reason). Interestingly, when asked by Isaacson why Apple won't try to enter the social networking biz again, Jobs replied "Maybe because Mark did it so well. There's not a need for a whole lot of social networks and Facebook has got it licked."
At the end of the day, these are both great men, who we should admire for inventing the great computers and means of communication we have today.