I have been in marketing for more than 14 years and have worked with quite a few large brands. The big brands (and the medium sized ones too) use agencies who choose their channels.
To get any traction you will need to get into groups like BBDO, Wieden + Kennedy, Grey, Ogilvy, The Martin Agency, TBWAChiatDay, etc. And you will never be able to pitch to their clients. Period. But they might listen.
But be warned though, they are somewhat closed to new platforms. They have been burned in the past and are not super thrilled about gambling with their customer's budgets. If they don't perform, they get replaced. I'm not saying it can't be done, but prepare yourself for a hard road and make sure your pitch is well practiced and targeted at them and what you can do for them and their clients (i.e. ROI.) It needs to shine!
Good luck. :)