By Gerry Shih SAN FRANCISCO (Reuters) – Twitter, whose shares had soared more than 150 percent since their November debut on Wall Street, stunned investors this week with a one-two punch of bad news: near-flat user growth and a drop in engagement among its 241 million members. To assuage fears that the companys growth is plateauing, and to justify its rich valuation – still at $28 billion even after a 24 percent drop on Thursday – Chief Executive Dick Costolo must now convince investors that he can quickly make the right moves to broaden Twitters appeal. Clearly weve seen that other social sites, even ones that are much bigger, have kept growing by adding new features and new functionality. So far, Costolo has asked for time to implement a series of changes that, while not directly lifting Twitters user growth, will help the service gain traction over several quarters. We dont think we need to change anything about the characteristics of our platform, we simply need to make Twitter a better Twitter, the CEO said.