I don't understand that at all. I assume your point is that Google only makes money from "ads", and that makes it a single source of revenue despite being well-diversified advertising products across a big segment of the tech industry (Youtube ads aren't the same as search ads, which are different from Android ads, etc...)
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?
Facebook has ads and Facebook Credits. S-1 shows about 18% of Facebook revenue comes from Facebook Credits, so Facebook is better than Google in "diverse source of revenue" metric.
That's fair; I didn't realize the fraction was so high. So, I guess the question then would be do freemium sales by affiliated apps constitute a meaningfully "diversivied" revenue stream? I'd argue that they don't, honestly. Downturns in the market that reduce eyeballs will hit them both. Both scale more or less directly with disposable income in the facebook user demographic.
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.
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?