I never understood why people think they are positioned against FB.. Google and FB don't solve very many of the same problems.
They don't, but they share one common goal: Get money from advertisers, who are targeting people looking for some thing.
This "some thing" is obvious on Google, its users are in buying mode even before entering the query term.
Now, if (and that's a big if), FB can connect advertisers and buyers, it'll slow down Google, because people will have spent their money with FB advertisers.
Google and FB customers are their advertisers. They might do completely different things but their revenue source is similar.
I personally don't believe in this. Facebook doesn't share the same easiness of monetization as Google, and it has some huge issues of making its visitors enter buying mode.
But there you go... even if I disagree, that's why some people think of a FB vs. Google fight. Follow the money :)
Google search ads EXCEL in "Demand Fulfillment" - say you want need a plumber now since your toilet is overflowing. I don't think Facebook will ever beat Google here. Google sort of created this niche where they were better than any competitor that had ever existed.
Facebook, on the other hand, is better at demand generation. Consider advertisers who want to reach teenagers who would want to buy videogames - a new game is coming out, and they want you to know about it. Before, you'd have to find the TV shows that they would watch, or billboards that they would see, or queries that gaming-teens would search for, and advertise there. These are all competitive and difficult proxies for the audience you want to reach. Facebook is a much better place for that than Google.
In fact, you can imagine synergy between FB ads and Google ads. FB tells people, "you should listen to this band that you haven't heard of", and when you Google that band name, you see ads for places to buy tickets and albums.
Google has money in abundance, they're putting millions on social projects, hoping to have some of FB's thunder (and data). While I don't know if Google will ever succeed, every social feature added by Google is a point of conflict with FB, thus feeding the "there's a war going on" opinion of some people.
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).
The secret of ad dollars is that not all visitors are equal. People who use google search are already looking for something, often something to buy, such users are the most susceptible to ads because the ads can actually be helpful. In contrast, ads inserted into the UI of a social networking site are far less valuable. In order for facebook to compete with google they'd have to reinvent themselves substantially.