Alternatives to AdWords

Just wondering what everyones thoughts are on alternatives to AdWords.
Adwords is performing great, no doubt, but we’re thinking one way to bring in extra clients is to start bringing them in from difference places.
One idea I had was using Yahoo’s PPC – I’m wondering what people’s experiences are with it.
Another thought was Facebook, who I know are also doing advertising now.
How good/bad is it and would it be worth investigating?

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