Twitter CEO Dick Costolo is just one of many ex-Googlers at Twitter joi ito Earlier this year, as part of an on-going dialogue between Google's corporate development team and Twitter, Google floated the idea of an acquisition with a price around $2.5 billion, we've heard from a source close to an executive involved in the talks.

We'd caution that this was not a formal offer – more of a casual offer to offer.

The overture was refused – even characterized as "insulting."

A second source, a Twitter shareholder, tells us he was told Twitter turned down a $4 billion offer about three months ago. This source does not know who the buyer was.

Both sources speculate that Microsoft may have made an offer too. This makes sense to us. One reason Google would buy Twitter is to keep it out of Microsoft's hands.The logic goes the other way, too. If and when Twitter truly goes "in-play," we fully expect those two companies to bid against each other aggressively.

Twitter is said to be seeking more funding at a $3 billion valuation. If the company is turning down $4 billion offers, that figure makes sense.

As we all digest this news – GOOGLE TRIED TO BUY TWITTER! – there is one larger to point to remember: People at companies like Twitter, Google, and Microsoft are talking to each other all the time. Informal offers are a part of everyday life on corporate development teams at a companies as large as Google and Microsoft. For startups like Twitter, this kind of talk is excessively common as well. During its first five years, no fewer than 11 companies talked to Facebook about a possible acquisition.

Google declined to comment on this story.