Twitter Inc. shares TWTR, -1.75% surged 8% in after-hours trade Thursday, after the microblogging site said Chief Executive Dick Costolo is stepping down on July 1. The company said co-founder Jack Dorsey would serve as interim CEO while the board searches for a permanent replacement. The company reaffirmed second-quarter guidance and said it sill expects revenue to range from $470 million to $485 million. Stock-based compensation costs are expected to range from $190 million to $200 million, the company said. Shares are down 0.1% in 2015, while the S&P 500 has gained 2.4% on the year.