Less letter-writing, more legislating. U.S. senators don't need to politely ask Zuckerberg to stop this app--if they want to put a stop to it, they should be willing to actually make a law against it.
I’m sure Markey would be happy to legislate against it. But thanks to the filibuster he would need to get 49 Democrats and another 10 Republicans on board before that would accomplish anything.
