SAN FRANCISCO (AP) -- Twitter Inc., the Internet service for short messages, is developing a penchant for keeping its CEOs' tenures relatively brief, too.Evan Williams on Monday became the second Twitter co-founder to step aside after a two-year stint, making way for a colleague considered to be better suited for steering the next phase of the company's rapid growth.Known simply as (at)Ev on Twitter, Williams intends to focus on product development while continuing to work for the company.The change in command at the privately held company turns over Twitter's leadership to Dick Costolo, an early investor who was hired as chief operating officer a year ago.Since Costolo's arrival, Twitter has moved into larger offices in San Francisco to accommodate a payroll that has swelled from a few dozen people to about 300 employees.Costolo, 47, also has played a key role in devising an advertising for

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