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If you're the application developer and they're the platform owner, you have to know death can come at any moment: Create a popular, simple application, and the platform owner might just rip you off in their next release. It's happened to Max Levchin's Slide, maker of the popular Facebook widget Top Friends. With its latest profile redesign, Facebook now allows users to specify which friends they'd like to display to profile visitors. (See how Facebook's version works in the image above and you'll note that with the friends I've selected, my goal is to intimidate profile visitors with my powerful connections.) Before you feel too sorry for Slide, note that this is a feature MySpace has long offered. Slide, seeing that Facebook lacked it, promptly cooked up Top Friends, which filled the void. Top Friends is Slide's second most popular application with nearly 1.5 million daily active users. On the strength of those user numbers, Slide has raised $50 million in a recent financing round, and is opening an ad-sales office in New York. We asked for Slide's reaction. They were surprisingly chipper!

"Yes, we view this feature as directly competitive to a relatively small part of our Top Friends functionality," Slide's Keith Rabois told us. "A developer on any platform must expect that their popular, but simple, features will be absorbed into platform over time."

But none of this has the salesman in Rabois down. He goes on:

You can see that Top Friends has a very large number of complex features that have a complicated back-end (Awards, Visual Personality, Music, world-class skins) — we expect those will continue to be long-term strategic advantages over other large developers and the platforms themselves

The secret of social networking revealed: world-class skin! We always suspected as much.