The jury deadlocked on a fourth count of conspiracy against the woman, Lori Drew, 49, and the judge, George H. Wu of Federal District Court, declared a mistrial on that charge.
Although it was unclear how severely Ms. Drew would be punished — the jury reduced the charges to misdemeanors from felonies, and no sentencing date was set — the conviction was highly significant, computer fraud experts said, because it was the first time that a federal statute designed to combat computer crimes was used to prosecute what were essentially abuses of a user agreement on a social networking site.
Under federal sentencing guidelines, Ms. Drew could face up to three years in prison and $300,000 in fines, though she has no previous criminal record. Her lawyer has asked for a new trial.
In a highly unusual move, Thomas P. O’Brien, the United States attorney in Los Angeles, prosecuted the case himself with two subordinates after law enforcement officials in Missouri determined Ms. Drew had broken no local laws.
Mr. O’Brien, who asserted jurisdiction on the ground that MySpace is based in Los Angeles, where its servers are housed, said the verdict sent an “overwhelming message” to users of the Internet.
“If you are going to attempt to annoy or go after a little girl and you’re going to use the Internet to do so,” he said, “this office and others across the country will hold you responsible.”
During the five-day trial, prosecutors portrayed Ms. Drew as working in concert with her daughter, Sarah, who was then 13, and Ashley Grills, a family friend and employee of Ms. Drew’s magazine coupon business in Dardenne Prairie, Mo.
Testimony showed that they created a teenage boy, “Josh Evans,” as an identity on MySpace to communicate with Sarah’s nemesis, Megan Meier, who was 13 and had a history of depression and suicidal impulses.
After weeks of online courtship with “Josh,” Megan was distressed one afternoon in October 2006, according to testimony at the trial,...