Ah, this is a very good point you bring up here. Had not thought in those terms, but you are very right. Advertisement, however tends to be very pan-platform if you will. Even more so these days, integrated, vertical marketing seems to be the name of the game. The big money will continue to make ad buys on all platforms that deliver eyeballs, not just one or the other.
Yes, because the more venues you broadcast your ad, the more chances you have to find a customer.
But not quite, because of one thing: ROI.
On the web, you can easily measure the return of your investment. Match your spent money with the server visitor logs, and you know instantly if your campaign is profitable. With "brand" advertisements, the game is different, you can't see immediately that 1 dollar spent on ad generated 2 in sales. But it's a tried and proven method of advertisement, basically all we had until the internet.
Enter the web, you can reach a precisely targeted audience, and easily measure the return.
If you know Google delivers more ROI than Facebook, you're going to spend your money on AdWords. This is specially true for the long tail of ad money, businesses with smaller budget for ads. Google has this market locked tight, anyone can start an AdWords campaign, and the truth is that everybody does.
Don't get me wrong, Facebook makes money from advertisement, but they will never (in its current incarnation) match Google's offers: AdWords, Analytics, Webmaster Tools, Optimizer, etc.
What Facebook has to do to connect advertisers and consumers is so much more complicated than Google. Build a "social graph", find their preferences, match with their friends, plus putting their users into buying mode. BUT, if delivered, it's way more powerful, because the product has been proven with your circle of friends and with stuff that you have actually said you like (Facebook knows a lot about everyone).
