Here's a new problem for the people running popular online properties like YouTube and Facebook to complain about: Ad agencies love using those sites to market their clients, but advertisers are beginning to realize they don't have to spend a dime to do so.Even when they do, the platform companies aren't the ones who see the profits. Lonelygirl15's creators, for example, make most of their money selling product placements in their videos. YouTube doesn't get any cut of that revenue. A top exec for a major interactive agency told me yesterday: "I keep telling my guys I"m going to do a contest next year to see who can come up with a media plan that costs $0, outside of our fees, of course." It shouldn't be too hard. Marketers create free Facebook pages for all kinds of brands. It's just as free to upload a YouTube video. And if an agency uploads one as clever as the above American Express ad, and its sequel, below, the agency won't need to pay anybody to promote it.