Facebook is set to announce the hire of Sheryl Sandberg, VP of vice president, Global Online Sales and Operations at Google, as its new COO, reports sister pubs ATD and WSJ. An official announcement is slated for this afternoon. Sandberg, a six-year Google veteran, is a key player in Google’s ad sales, i.e. the bread and butter of the company. It’s no big mystery why Facebook, still looking for the right ad strategy, would hire her. CEO Mark Zuckerberg told WSJ: “Things are growing so quickly here at Facebook… Sheryl has I think the most relevant industry experience we could find at this point.”

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The company has also been under pressure to bring in more “adults.” It recently lost chief revenue officer and erstwhile COO Owen Van Natta, who is out looking for a place to be CEO, and there’s been a perceived experience gap, as well as an obvious #2 to Zuck. Sandburg would seem to satisfy all of these issues solidly.

For Google: It already knows that talent loss is one of its big problems (though this looks like a big one), and with its go-go growth days seemingly behind it, don’t expect the talent drain to stop anytime soon; various “the next Googles” beckon.

The release is now out here: “Facebook today announced that Sheryl Sandberg will join the company as Chief Operating Officer starting Mar. 24, 2008. As COO, Sandberg will be responsible for helping Facebook scale its operations and expand its presence globally. Sandberg will manage sales, marketing, business development, human resources, public policy, privacy and communications and will report directly to Facebook’s CEO Mark Zuckerberg.”

NYT has a few more interesting quotes from the key parties: Zuckerberg: “Communicating what we are about clearly is an important thing for us to do… We can do that better, and Beacon showed that, as did a handful of other things.” Sandberg: “I’ve loved being part of the process of helping to build Google (NSDQ: GOOG). The opportunity to help another young company to grow into a global leader is the opportunity of a lifetime.” The report also notes that Sandberg will be replaced at Google by David Fischer, VP of online ad sales and operations.