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White-hot microblogging startup Twitter is growing at a booming pace, gleefully tweeting today that it just hired its 300th full-time employee as it continues a hiring spree that began last spring.

“We just hired our 300th employee. Wowza!,” tweeted Twitter’s recruiting arm, JoinTheFlock around noon Pacific time.

Since it began its meteoric growth, Twitter has nearly tripled in size, adding 400 people since June. It’s a massive rate of addition, considering Twitter had only 110 employees at the start of 2010.

In contrast, it had only 22 employees on its roster at the start of 2009 and had moved into spacious new headquarters—the former office of Bebo, another social-networking startup—in the SoMa neighborhood of San Francisco by the middle of November 2009.

Now, however, the company appears to be outgrowing its current digs. The company told the San Francisco Chronicle last month that it was on the hunt for new offices with at least an additional 200,000 square feet.

“Based on our current growth trajectory, we can’t stay in the two floors that we currently have. So at some point, we’re going to have to look at other options,” Twitter spokeswoman Carolyn Penner told VentureBeat today.

But where exactly that may be remains to be seen. In appearances with San Francisco Mayor Gavin Newsom, company cofounders Ev Williams and Biz Stone have expressed their commitment to staying in the city of San Francisco.

Williams blogged on October 4 that the company had reached 300 employees, but Twitter clarified those numbers to TechCrunch today, saying, “We had 300 full-time + contractors + interns at that time. Now we have 300 full-time employees.”

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