Facebook sells ad space to people on their chat platform that have little to zero intent to buy. The last time I saw a company (AOL) try to stuff ads in a chat client... well we all know how that went.
Facebook have a shitty ad product and it still generates $4b p.a and climbing, imagine how good they would be if they actually figured this stuff out
But... how is facebook any different? They have ads. And... more ads, as far as I can see. What's the "more than one" source of revenue you're referring to?
Normally you talk about diversification in the sense of risk management: i.e. "It's OK if the social media market tanks because we still sell phone service." (or whatever: hardware, concert tickets, coal mine permitting services, etc...).
From that perspective, both Facebook and Google are very exposed. Though if anything I'd still say that Google is better situated due their presence in pretty much all of online advertising. As long as there is anything worth advertising to someone on the internet, Google has an answer for that.
HW: Google Search Appliance, Google Nexus SW: Google Earth Pro, Google Sketchup Pro SaaS: Google Docs for Business, Google API's (Maps, Google+) Paas: Google App Engine Ads: Google Ads
I know that 96% of GOOG's 2011 Revenue came from Ads, but GOOG is at least trying other markets. I personally wouldn't be surprised if FB released a phone.
Please also consider a counter case, what if $4b revenue i.e. roughly $4 per user per year, is the best you can have for such a product. And the potential for growth of no. of users is not high, then what?