If a story from Yahoo Financial News is to be believed, some time earlier this year Google played with an idea of buying Twitter as w whole, with a price somewhere around $2.5 billion. The offer was not official but there was obviously some communication between the two companies.
According with the same source though, Twitter felt insulted (cheap, cheap…) and declined. Another offer came allegedly about 3 months ago, this time for $4 billion, with the same effect. Microsoft also tried its luck, although I imagine beside the obvious interest, a huge part of the premium offered are due to each of the giants efforts to ensure the other side doesn’t get Twitter, more than the actual interest of owning Twitter for themselves.