Excellent article Alex! I'm actually rather amazed that this connection wasn't more readily apparent to me. Nonetheless, I think you didn't address the distinction between B2C and B2B business models. Excel is both a B2B and B2C product, where Facebook almost exclusively serves just B2C (who among us solicits business deals using Facebook?).

If your SaaS is targetting B2B then Facebook is a non-issue. It goes almost without saying that Facebook has no teeth to eat your lunch.