So I finally started an Instagram account and this is essentially the original facebook, just image-centric. And it is great. I bought some stock a few days ago and like others here, wish I bought more.
To add color here, I think my original premise is still true: FB's core competency is migrating users from social platform to social platform. To add to that, each migration should (1) add some value and (2) remove unwanted features.
A month ago when I posted that, I was unsure whether FB had successfully done so. Now I see that they have. Instagram is essentially the de-facto social network a la Facebook of 5-10 years ago, and actually the advertising is done much better.
Also there is a sticky effect. It's easier to migrate platforms under the same umbrella, you can import friends, contact information, media, etc.
Future platforms can be purchased (regulatory risk of course) or a copycat platform can be created. I don't think it's a bad bet...