Apple will sell somewhere around 12 million iPads in 2010, and hardware makers will ship about 350 million PCs to users around the world. But both of these newsworthy markets are dwarfed by the user base of a single online service, Facebook, which counts more than 500 million members and emerged in 2010 as the single most important online destination in the world. Of course, Facebook is famous—perhaps infamous—for other reasons: It's emerged as the single biggest personal-privacy hole online as well.

How the service balances the not-so-subtle difference between serving the needs of its users while simultaneously working explicitly to share their private data with advertisers and other third parties is what makes Facebook so intriguing. In fact, aside from Apple's antagonistic relationship with its biggest customer base (i.e., Windows users), there's little precedence for a company like Facebook.

Facebook was in the news many times in 2010, but rarely for positive reasons. Early in the year, the company's then-new privacy settings raised questions about the company's motives, which involved an (again) Apple-esque "technological lock in." In fact, privacy was a constant concern for Facebook users throughout 2010. And when the company overhauled its privacy settings yet again in October in the face of rampant criticism, privacy advocates were quick to point out the many remaining shortcomings.

There was some good news, however. Facebook partnered with Microsoft on the Office Web Apps-based Facebook Docs in April, for example, and its Microsoft partnership extended to Bing search results integration in October. In November, the company announced a revamped Messaging service that I think sets the stage for the next generation of communications, in which email will be de-emphasized for more immediate forms of electronic communication. Here, Facebook makes even Google look old-fashioned.

Facebook cofounders Mark Zuckerberg and Dustin Moskovitz also announced in January that they would be giving away half of their fortunes as part of a philanthropic campaign started by activist billionaires Bill Gates and Warren Buffett. That this money came, in part, from allegedly stealing intellectual property from the individuals who actually started the original Facebook service, and then from routinely harvesting the private data of its users does, of course, somewhat diminish the gift.

But again, this is why Facebook is so fascinating, and why this company—and not Apple, Google or Microsoft—was the subject of a 2010 blockbuster Hollywood movie, The Social Network. As its eager users will tell you, no other service provides a way for them to so seamlessly and easily keep up with their friends, family, and other loved ones. And it is this essential service that makes Facebook so compelling, even with its inscrutable and user-antagonistic privacy settings. And that's really how we know that Facebook is so important: If it wasn't, few would put up with its rampant and obvious breaches of trust.

For myself, Facebook has emerged as an essential online service, one that is as important to my personal life as Gmail, Google Calendar, and Microsoft Word are to my professional life. You may not like it; you may in fact loathe it. But chances are you do want it. Facebook is here to stay.