Does Yammer represent the last acquisition of its kind? It’s an interesting question discussed by a bunch of tech heavyweights here, including Marc Andreesen. Many comments touch on many different points. It’s well worth the read.
For my part, Amazon, Apple, Facebook, and Google have concurrently made it easy and difficult for others to innovate on their platforms. Open APIs and SDKs encourage innovation. However, as we’ve seen with StockTwits and others, those improvements can easily be incorporated into those very platforms.