While Google ads are more effective, they're often more expensive (at least in the hosting industry). And as has been said, they can't be as precisely-targeted.
I have two problems with Facebook ads: I find that their monitoring and reporting tools are very un-intuitive, and their results are simply not accurate! It's better to monitor results in Google Analytics, as they seem closer to the reality of visits/conversions.
If we're talking about promoting web hosting products like VPS or dedicated servers, then Google AdWords. If for shared hosting, then Facebook.
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