[piguy314]

The typical Price-Earnings ratio for a tech company is about 20, which means twitter would only need to generate 1/20th of that amount per user to justify it's valuation.

EDIT: After re-reading your comment it seems you've already incorporated this concept with your discount rate, I would argue that your conversion and discount rate are both too high. Premium conversion would probably be something on the scale of 1-5 percent, and the discount should be 5 percent (1/20).