Instagram was FB's biggest competitor... huge growth and huge adoption amongst young groups of friends. FB was smart to buy them.
Pinterest... I'm less bullish on. I can see how they can make money with affiliate commissions on products and such. But I just don't see Everyone using this. Pinterest at $1.5billion might be over-valued for where it's at. But then again, I don't have privy to their internal numbers (ie., traction, metrics, retention, etc). But, I can see Pinterest be worth a lot in the future, just right now I'm not sure about the $1.5b.
FB, Instagram and maybe Pinterest are amazing companies with crazy traction. They're all riding on the huge distribution channels of our day (web, mobile, social) and have reached or are close to reaching the 100+million users mark (for FB is the 1+billion user mark). Any service that can garner 100s of millions of users can find a way to monetize. How much in monetization is another story, but I wouldn't bet against them to create some substantial value.