Like investing in Apple after learning about the death's of other mp3-player-companies?
That is not Facebook, it sells nothing other than the network, much like MySpace used to. If yours friends drop off Facebook, you will drop off too. That is not same for Apple. Apple users will continue using the products even if their friends do not. Its a personal device.
This works both ways. I use Facebook because my friends are there. I use Apple products because I like the products. Switching social networks is has a high friction point - all my friends need to move. Switching phones is my own choice and easy to do.
Look how much easier it is for Google to gain half the smart phone market compared to how (relatively when compared to Facebook) unsuccessful they've been at getting "social network" market share. I think a very good argument can be made that Facebook's position is more defensible then Apples.
A lot of the value in Apple's products comes from the ecosystem: iTunes, the App Store, and so on. Don't discount the value of a platform on which a large number of developers are incentivized to build.
It is now, but the iPod is a good portion of what made all that possible.