Shares of Pandora plunged Monday as news that co-founder Tim Westergren was returning as chief executive failed to ignite confidence in the group’s long-term prospects. Westergren, who helped found the Internet radio service in the…

Shares of Pandora plunged Monday as news that co-founder Tim Westergren was returning as chief executive failed to ignite confidence in the group’s long-term prospects. Westergren, who helped found the Internet radio service in the…