Washington: Facebook celebrated its fifth birthday on Wednesday with 150 million guests, but no clear path to profitability.

Chief executive Mark Zuckerberg, 24, who co-founded the social networking website with two Harvard University roommates, said he was “humbled” by the phenomenal growth of a website, which began as a way for his fellow Harvard students to stay connected and has blossomed into a worldwide network that has dwarfed rival MySpace.

“Building and moving quickly for five years hasn’t been easy, and we aren’t finished,” Zuckerberg said.

“The challenge motivates us to keep innovating and pushing technical boundaries to produce better ways to share information,” he added.

One challenge Facebook is yet to overcome is how to make money.

Unlike other Web giants, Facebook is yet to prove how it is going to translate traffic into cash.

“There’s no significant visible source of revenue other than investors,” said Silicon Valley analyst Rob Enderle.

“They need to grow out of their childhood really fast. They can’t be a five-year-old (child) anymore,” he said. “They have to grow into adulthood and adulthood means paying the bills.”