According to sources close to the company - Microsoft’s bankers have been putting out subtle signals to Facebook to see if it would be open to a full buyout of Facebook.Microsoft already invested $240 million, back in October 07 in Facebook, the social-networking site... hence the wacky current $15 billion valuation.No word on what Facebook’s reply was, although Harvard University dropout, 23 year old Facebook CEO and Founder, Mark Zuckerberg has long said he’s not interested in selling the company.Zuckerberg owns a 20 percent share of Facebook, which is now valued at $3 billion. Accel Partners, the venture capital firm that invested $12.7 million in May 2005 and owns 11 percent of Facebook, now holds stock worth $1.65 billion. Facebook spokeswoman Brandee Barker declined to comment on the report.