Relying on Facebook is pretty risky - but so is doing a startup. The bigger picture is that they (and I) believe that applications built on SOME social platform are going to be the future of web applications. We're barely seeing the hint of that future today, and making a bet early could pay off big or we could all be wrong.
That would be an interesting and depressing future, with companies like facebook dictating the rules of the internet. I certainly hope it doesn't come to that.
I've always thought that news sites like digg would become the eqivalent of the Web OS, the gateway to the internet. I can see Bacebook filling that space easily though, now that I think about it. The difference there is that digg is pointing users to other places on the internet, while facebook in that case actually BECOMES the internet for those people.
It'll certainly be interesting to see how it all pans out in 10 years.
I think a modularized web will be a big part of what's coming up.
Let's also not forget that all other social networks are going to open up, creating a massive opportunity for Slide that probably far exceeds what Slide makes on Facebook. $500 mil may not be such a high valuation when you think about it.
But look at facebook right now; its arguably worse with the apps. Companies like Slide hijack your news feed and spam your friends. That's uncool and it puts me off.
A lot of those spamming tactics are how apps get so much exposure, but I don't see Facebook allowing that kind of stuff for very much longer.
My point is, relying on Facebook as your provider is pretty risky. I wish you continued luck, though.