Facebook gave preferential treatment to Zynga only after they proved that they were the most popular. They had to fight their way there - it didn't start that way. It's still a bit dangerous to do so though because users' tastes may change, so good on Facebook for removing their special status.

As you said, Google+ had no such information, and gave companies special status out the gate before proving their popularity. If Facebook did the same thing, their top apps would probably look a lot different.