If he doesn't respond to that, you could get more extreme by posting your own status and tagging him in it: "I've been working for @[friend] for months, and he hasn't paid me or responded to my emails. Anyone have any suggestions?"
Phone calls, in-person visits to his office, and emails to other friends or employees at his company will also be effective. It's very easy to ignore a personal email. It's very hard to ignore something that starts to affect your reputation and peace of mind.
Other than that, you might have some luck selling the debt to a collection agency.
You can, of course, always sue him, but if you don't have a pretty tight contract, it would be more trouble than it's worth.
