Tumblr was founded in 2007. That makes it nearly 4 years old. Twitter was founded in 2006. That makes it nearly 5 years old.

At what point in their "Humble Beginnings" do you suggest they start thinking about making some money and becoming self-sustaining?

Somewhere along the way they need to focus on being able to provide for themselves. Otherwise it's a giant ponzi scheme where all you get are investors investing in a product that won't return their investment.