Those rumors about Google‘s Facebook-rivaling social network just got a whole lot more interesting. A former Facebook exec has been quizzing contacts inside Google and discovered that it’s all real, with large numbers of Google staff busy on task.

The leak comes from Adam D’Angelo, cofounder of Quora (a cloud-sourced community question-answer forum), and it’s interesting as he was formerly chief technology officer at Facebook. Given the size and complexity of Facebook’s operations, this was no small job, and probably places D’Angelo on the contacts directory of a significant number of colleagues in competing high-tech businesses. In other words, when D’Angelo says he spoke to “reliable sources,” we’re inclined to believe him.

Digg’s founder Kevin Rose was responsible for the first part of the rumor, and it piqued D’Angelo’s interest enough that he did some of his own digging–turning up some data from his inside sources. He learned that it’s “not a rumor” and there are a “large number” of folks inside Google working on it, and this news is convincing enough that he’s “completely confident” about its veracity. Unlike other projects inside Google, “this is a high priority” one.

While interesting, these points don’t necessarily teach us anything new about the project. But D’Angelo also learned that Google really feels it dropped the ball on the whole social networking phenomenon, particularly where it concerns Facebook. “They had assumed Facebook’s growth would slow as it grew, and that Facebook wouldn’t be able to have too much leverage over them, but then it just didn’t stop, and now they are really scared” is the most telling phrase, because it implies naivety about several aspects of the world inside Google’s management. Firstly, by the very nature of social networking it’s a phenomenon that tends to grow virally–the more people on Facebook, the more people get attracted to Facebook since more of their friends are already there: For Google’s team to not understand this is surprising (and possibly explains the weak success of Google Buzz). There’s also a hint in here that Google’s management doesn’t have a good grip on how people are interacting socially on the Web, and that they underestimated how people liked the friendly interactivity offered by Facebook–another indication why Buzz, with its slightly obfuscating interface, isn’t as much of a success as it could be.

Essentially, it seems that Google’s management had assumed that Google Buzz would leverage off Google’s enormous reach online, and its brand name, and easily compete with Facebook. This proved such a disastrously bad move that the new Google social net will actually be modeled “off Facebook.” While we don’t know exactly what this means, we can guess that Google may be intending to emulate Facebook’s interface more than the way the site’s social interaction tricks work, since Buzz was actually an attempt to do this.

To keep up with this news, follow me, Kit Eaton, on Twitter.