U.S. stock indexes finished mostly flat on Friday, with the Dow snapping a six-day streak of record closing highs. Shares of Twitter Inc. , the social media company that has nearly tripled in value since going public in early November, fell 13 percent today to close at $63.75 as the the New York Stock Exchange’s most actively traded stock. The tech-heavy Nasdaq fell 0.25 percent, with leaders like Apple off 0.7 percent at $560.09 and Facebook Inc. down about 4 percent at $55.44. The Nasdaq has soared 37.7 percent in 2013, making it the best performer among the three major U.S. stock indexes. Both the Dow and the S&P 500 wrapped up a second straight week of solid gains. The S&P 500 posted its best two-week period since July, while the Dow marked its best two weeks since June 2012. The S&P 500 has jumped 29.1 percent this year, on track for its best year since 1997. The Dow has climbed 25.8 percent this year to position it for its best year since 1996. The Dow Jones industrial average fell 1.47 points or 0.01 percent today to end at 16,478.41. The S&P 500 dipped 0.62 of a point, or 0.03 percent, to close at 1,841.40. The Nasdaq Composite dropped 10.59 points, or 0.25 percent, to end at 4,156.59. Friday’s dip also halted the S&P 500’s run of four record closing highs in a row. For the holiday-shortened week, the Dow gained 1.6 percent, the S&P 500 added about 1.3 percent and the Nasdaq advanced roughly 1.3 percent. The U.S. stock market was closed on Wednesday for Christmas and stopped trading early on Tuesday.