WASHINGTON — Messages posted on Facebook and Twitter or sent in e-mails can be tasteless, vulgar and even disturbing. But just when do they cross the line from free speech to threats that can be punished as a crime? As the Internet and social networks allow people to vent their frustrations with the click of a mouse, the U.S. Supreme Court is being asked to clarify the First Amendment rights of people who use violent or threatening language on electronic media where the speaker’s intent is not always …