We’ve already said what kinds of hurdles Facebook faces with a $100 billion valuation. But what if shares trade near the low-end of the spectrum, say for $28, and don’t bounce on the first day of trading? What kind of prospects does the stock offer then?

According to the numbers crunchers at EVA Dimensions, a $28 share price means the market thinks Facebook will grow to sales of just over $50 billion in the year 2021. That’s still an impressive 30% annual compound growth rate, but it seems much easier to achieve than the $68 billion in sales implied by a $100 billion valuation. Those sales figures assume that Facebook’s margins don’t deteriorate badly from here. (Facebook, it turns out, earns amazing profit margins when you consider its capital costs. Last year it earned $1 billion on just $3.7 billion in sales— what EVA Dimensions calls “monopoly-like margins.”)

We’ve said before, Google (GOOG) once faced the same hurdles but eventually grew into its valuation, and took stock investors on a ride from $85 to $650 along the way.

There’s no easy take on Facebook stock. If you believe it can continue its torrid sales growth without killing margins, the stock looks like an amazing opportunity. If you doubt it can continue boosting sales, stay away.

Read last month’s Fortune story on Facebook’s valuation here.