Brad Stone

When Facebook signed up its 100 millionth member last August, its employees spread out in two parks in Palo Alto, Calif., for a huge barbecue. Sometime this week, this 5-year-old startup, born in a dorm room at Harvard, expects to register its 200 millionth user.

That staggering growth rate — doubling in size in just eight months — suggests Facebook is rapidly becoming the Web’s dominant social ecosystem and an essential personal and business networking tool in much of the wired world.

Yet Facebook executives say they aren’t planning to observe their latest milestone in any significant way. It is, perhaps, a poor time to celebrate. The company that has given users new ways to connect and speak truth to power now often finds itself as the target of that formidable grass-roots firepower — most recently over controversial changes it made to users’ home pages.

As Facebook expands, it’s also struggling to match the momentum of hot new startups like Twitter, the micro-blogging service, while managing the expectations of young, technology-savvy early adopters, attracting mainstream moms and dads, and justifying its hype-carbonated valuation.

By any measure, Facebook’s growth is a great accomplishment. The crew of Mark Zuckerberg, the company’s 24-year-old co-founder and chief executive, is signing up nearly a million new members a day, and now more than 70 percent of the service’s members live overseas, in countries like Italy, the Czech Republic and Indonesia. Facebook’s ranks in those countries swelled last year after the company offered its site in their languages.

All of this mojo puts Facebook on a par with other groundbreaking — and wildly popular — Internet services like free e-mail, Google, the online calling network Skype and e-commerce sites like eBay. But Facebook promises to change how we communicate even more fundamentally, in part by digitally mapping and linking peripatetic people across space and time, allowing them to publicly share myriad and often very personal elements of their lives.