Who is left at Twitter?

November 14, 2011, 9:22 PM UTC

FORTUNE — Why is everyone leaving Twitter? The product’s growing more popular by the day — it’s now generating 250 million tweets, a jump from 90 million in September of 2010. As of mid-October, the company’s value was some $8 billion, according to CEO Dick Costolo. And yet, a year after the management shakeup that put founder Jack Dorsey back into the chief product role and Costolo in the CEO spot, employees continue to leave.

Consider this:

— Head of communications Sean Garrett announced his resignation last week, November 9. After he arrived, the Twitter communications department went from zero people to eight by the time he left.

— Mike Abbott, VP of engineering, announced last month on October 13 that he would be leaving and moving to his a role as Entrepreneur in Residence at Benchmark Capital.

— Chief scientist Abdur Chowbury, who was responsible for implementing Twitter’s search recommendations, confirmed that he would be leaving to work on Alta Vista School, an independent San Francisco elementary school he co-founded in mid-September.

— Pam Kramer, who was hired in July as Twitter’s first VP for consumer marketing, left the company after just three months.

— One of the most recent departures is Loren Britcher — creator of iOS app Tweetie — who tweeted, “Today was my last day at Twitter. Taking some time to figure out what’s next. Really proud of the way the team has grown.”

— Two of the company’s first VC investors, Fred Wilson of USV and Bijan Sabet of Spark Capital, left the board in mid-September.

The unanswered question is, of course, why are the top employees leaving? Despite rocket growth, the trouble @Twitter chronicled by Jessi Hempel in Fortune‘s April issue seems to be ongoing.

In a statement, Twitter responded saying, “Twitter has experienced record growth over the past year, growing from 200 employees last summer to more than 700 people today. It’s natural that some employees may move on to other adventures as the company and its business continues to mature.”