Twitter has named former Pearson CEO Marjorie Scardino to its board after the microblogging service faced criticism that it lacked women in its top ranks. The appointment of Scardino, 66, is effective immediately, Twitter said Thursday in a filing. She led Pearson, the education and publishing company, from 1997 to last year. The addition of Scardino expands Twitter's board to eight people. The absence of women on Twitter's board was "a joke," Sallie Krawcheck, a former Bank of America executive, said last month at a Bloomberg conference. It showed Silicon Valley was no better than Wall Street when it came to female representation, she said. Also on Thursday, Twitter launched a new form of targeted advertising, allowing companies to deliver "promoted tweets" to customers who show interest in the brand through web browsing.