They are both very different, and both very good in the right situations.

Firstly, google can target users actively looking to buy,  with keyword searches. This is the bread&butter of google adwords.  These are high value customers, and generally there is strong competition, and the prices are high. If you have a good budget, this can build business.

Google can also target more latent demand via the content listings – it attracts people surfing not necessarily via a google search. This is a little similar to the facebook advertising.

You can target geographies, and keywords with google. Google targets the public internet.

Facebook doesnt really have a search feature, so all adverts hit people communicating with friends and family, so there isn’t as many hot/active users to target.

Facebook you can target very specific demographics, age/sex/marital status/location/job/education/employer/interests/groups/hobbies. It can get very specific. The more specific it is, the more targeted your spend is.

Facebook interestingly targets the private social web. The pages people are surfing aren’t generally available to google – its closed to facebook.

So there is something to be said for advertising in BOTH google and facebook. Obviously the adverts and calls to action are likely to be different.

I think google adwords is evil, but I’ll post about that seperately another day.