It looks like Facebook has set its IPO for May 18. The Wall Street Journal tries to be skeptical about Facebook’s business model, but several of the critical advertisers and agencies it quotes admit they’re increasing their spending on the social network. And a nifty comparison chart (what you can get in advertising for $1 million) hints at the ROI doubt on pretty much every kind of brand advertising. I don’t want to sound like a Facebook apologist – and certainly, its ad salespeople should answer their phones – but it’s always challenging to prove the value of brand advertising. Digital media should be more measurable, and online surveys and other techniques mean that testing ad effectiveness online can be cheaper than in traditional media. But it’s partly up to Facebook to start connecting those dots, and doing some of the media plan testing required. That, and prove that social marketing actually is more effective than display advertising.