How about a free seat sale...

Loads, whilst important, do not reflect how well the airline is doing. If the airline can make low loads work but the flights yield enough to make money then the airline is content.

They are however, at the moment penetrating many different markets. IAD-LAX has many loyal FF including those of Uniteds who deploy widebodies (B777s) during peak times on these flights to meet demand. Breaking into the market will always be the issue with Virgin America given that they're a relatively small airline with few partners.

I beleive the way to break into the market would be to inform the American population of the services onboard along with the great price with a massive marketing campaign.