Twitter has claimed it is "inevitable" the micro-blogging service will hit the 'one billion users' mark.

Evan Williams, co-founder of the micro-blogging site who recently stepped down as CEO, said Twitter currently has about 165 million members. Williams did not estimate when Twitter would reach the milestone.

Meanwhile, social networking rival Facebook has better odds of reaching that goal in the not-so-distant future. In July, Facebook announced that it had passed the half billion user milestone.

Zeus Kerravala, an analyst at Yankee Group Research, said Williams has set a lofty goal for Twitter, but that the firm does have a chance to meet it.

"A billion does seem a bit aggressive," said Kerravala. "But I do think, over time, social networking will overtake email as the most popular communication tool. In that light, one billion isn't out of reach. Younger people will choose Twitter and Facebook over email."

Email, according to Kerravala, is so... well, yesterday, while Twitter is more like a modern, digital firehose.

"Over the next five years, as school kids go to college and college kids enter the workforce, we'll see a rapid shift away from 'old people' communication tools like email and the phone," he added. "Twitter allows you to reach out to thousands of people at once."

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