The micro-blogging service has agreed to sublease a 31,000-square-foot office at 795 Folsom St., now used by another social-networking firm, Bebo Inc., which is owned by AOL.

Sarah Gavin, vice president for communications at AOL, said Bebo plans to move from the sixth floor to a lower floor "to take up a more appropriate size space in the same building."

According to Twitter's Web site, the company has fewer than 30 employees working in its current headquarters at 539 Bryant St. But Twitter also has 30 openings posted online for positions that include software engineers, customer support workers and office support staff.

Twitter officials did not respond to requests for comment.

Twitter usage has grown dramatically in the past year and the company now has about 54 million users. But the surge in growth has also been accompanied by intermittent service disruptions.

The company hasn't stated how it will generate revenue, but last month it closed a round of financing from several investors.

Bebo, with more than 24 million users, operates in offices about three times larger than Twitter's space. Bebo's biggest markets are in the United Kingdom, Ireland and New Zealand, Gavin said.

Commercial real estate activity in the South of Market area is picking up because there are many leases set to expire in one to three years, said Colin Yasukochi, a research director for Jones Lang LaSalle, a real estate service and investment management firm.

"If you find a good enough deal and it's worthwhile to move early, sometimes tenants will do that," Yasukochi said.

E-mail Benny Evangelista at bevangelista@sfchronicle.com.

This article appeared on page DC - 1 of the San Francisco Chronicle