1) Go to Google and type in "Plumber" (a search that is already localized to SF and shows plumbers near me right away).
2) Go to Yelp (or another similar site) and do the same search to find more review-based results.
3) Go to a social network...
Yes, there may be some cases where FB is better (for example, if I don't even know that there is some local service I should check out) -- but those seem pretty rare.
1) Buy some PPC ads via Google and maybe a few new customers will trickle in via searches
2) Do a groupon, possibly getting a huge influx of business BUT at a 50% discount and giving groupon half the proceeds
3) Pay for a Facebook "Flyer" targeted at San Francisco home-owners that have a dog or cat, offering them a 20% discount ...
I think when you look at it from the POV of the business doing outbound marketing, Facebook has more opportunities.
Of course, FB has to be careful not to turn the system into a huge coupon-book and alienating users by the hundreds.
If I search for "carpet cleaning in san francisco" on google I see 3 ads. I suspect that those ads are responsible for a fair amount of business to those companies, explaining why they pay google for the advertising.
The difference between ads on google and ads on a social network is that ads that are relevant to your search are actually often desired and far more likely to be used. If I'm searching for a carpet cleaner in san francisco then ads of such kind are useful to me. In contrast, ads embedded in social networking sites are distractions from the activities people engage on there. If some carpet cleaning company pushes an ad on me because they've discovered I live in the area and/or have pets I will more likely than not think "ugh, who are these assholes?" or perhaps "maybe I should install ad block". When I actually need a carpet cleaner I'm not likely to think "hey, I should use that company that keeps annoying the crap out of me on facebook", instead I'm likely to go to google, search, compare some reviews, and pick a company that seems decent and inexpensive.
Sure people who are actively looking for a service are more easy to sell to and I'm sure Google, etc. will make a lot of money from them. But they're only a small part of the overall market.
I still don't think it's a good long term investment, but they should be able to sell a lot of ads the next few years.
So if plumbers are like mechanics and you have to find one that's happy and good and trustworthy, you might actually be more likely to ask your friends than Google, especially if Google is a corporate spamwhore.