No, media control was not the key issue, or else Pacifica could be calling out mass rallies in New York, LA, New York, DC and San Francisco every weekend. In this particular event, there is little doubt that media was a key source of information, but it was only because people were primed because of anti-immigrant attacks that they listened and turned out.
Maybe owning more media would be a useful thing for unions, but as the Pacifica example emphasizes, owning an outlet doesn't get you much unless the message is right. And when the message is right, you usually don't need to own the media to get it out-- you usually can find others to carry it, as happened with the organizers of the LA march.
--- Nathan Newman