For a company that thrives on transparency, Facebook's advertising isn't very transparent. Check out your Facebook home page, note the ads on the right side, and tell me what companies they're for. Sure, the ads probably cite brand or company names in the headlines and images, but who are the companies who paid for those ads? Where will you end up if you click those ads? And an ever better question is, what will happen if you "Like" the ad--will you be giving information to a trusted brand or a spammer?
Now go to Google, conduct a search, and check out the AdWords on the right side. Who sponsors those ads? And where will you end up if you click on those ads?
Google advertising is transparent, and Facebook advertising is not. The difference is a single line of text: AdWords creates transparency by including a "Display URL." Within ad AdWords, advertisers can set different Destination and Display URLs, but the two must be within the same domain so that (in the word of Google's AdWords form), "users know what to expect when they click your ad." (The reason there is a difference between the Destination and Display URLs is so that advertisers can direct people to a specific page in their domain while displaying the much shorter root domain in the ad ).