Do you really think Facebook would stop innovating if it weren't for the threat of Google behind its back? Perhaps the timing is influenced by G+, but the actual introduction of those products are just evolutionary steps in Facebook's mission to make the world more connected.

It hires so many people (including hundreds of interns) because it wants to build a platform and a tool for people to use to keep in touch with each other. I don't think Facebook is playing from the same opening book as Microsoft.

IE is a different story because it is not a core product of Microsoft. Facebook is the core of Facebook (obviously), so it cannot just let the product atrophy like MS let IE.