Where is it? Facebook engineer Jeff Verkoeyen has quit to take a job at Google -- a striking reversal from the recent trend of employees moving the other way, forcing Google to pay seven-figure bonuses to keep them around.

The main reason: he's frustrated that Facebook still hasn't released the iPad app he worked on, even though it's been basically done for months.

Verkoeyen wrote on his blog today that he was the lead engineer on the Facebook iPad app back in January. He put a ton of time into the app.

It was feature-complete back in May, he writes, but Facebook kept pushing its release out another two weeks, then another. (Feature-complete doesn't necessarily mean "finished," but it's usually the last stage before a public beta test.)

Now, he thinks it "may never be released."

He wouldn't say why, but perhaps Facebook's rumored dispute with Apple is the reason. Apple is reportedly more scared of Facebook than any other company because it's worried that Facebook's "Web-as-platform" strategy might make the iPhone and iPad less relevant.

Or maybe it's just that Facebook is struggling with its mobile strategy. It didn't make any mobile announcements at F8 last week, and other engineers working on Facebook's mobile strategy like Chamath Palihapitiya and Joe Hewitt, have also left in recent months.

Facebook CEO Mark Zuckerberg was non-committal in an interview with CNBC Thursday, saying that Facebook would probably create an iPad app at some point.

Verkoeyen will be working on back-end mobile technology for Google's Android team. According to a timeline on his blog, he worked there for a few months in the summer of 2007. He's been at Facebook since June 2010, with some brief stints there before then.

Update: Verkoeyen has updated his blog post to remove details about the iPad app. He also called us to clarify that the frustration was his "personally." He said "It's no reflection on Facebook as a company, which is an incredible place to work." He had no further comment on the iPad app or Facebook's mobile strategy.