Facebook Inc. shares today are a lot closer to $13.80 now than in late May.

That's when I devoted a column to calculating what a fair price might be for the company's shares, coming up with $13.80. That column came a week after Facebook (FB) went public at $38 per share, and when those shares were trading for slight more than $33 per share. (Read my May 25 column, entitled "Facebook stock should trade for $13.80.")

In after-hours trading Thursday evening, following Facebook's first earnings announcement as a public company, the shares were trading below $24.

The reason to revisit my fair-value calculation today: Data in the newly released earnings report allow me to check whether the assumptions I used in that calculation appear to be too generous or too conservative.

Those assumptions include:

The key assumption on which Facebook's latest earnings report can provide a reality check, of course, is revenue growth. The company reported that its revenue in the second quarter of this year was 32.3% higher than in the second quarter of last year. That's slightly higher than the 25.6% annualized growth required for Facebook's revenue to hit the $11.58 billion I assumed in my late-May column.

So I re-ran my fair value analysis on the assumption that Facebook will be able to keep its revenue growing at this slightly higher rate. On that assumption -- which I will argue in a minute is generous -- then Facebook's fair value today would be $17.92 instead of the $13.80 estimate I reached in late May.

Better than the picture my analysis painted in May, but not a lot better -- since this higher estimate is still 25% lower than where the stock currently trades.

The reason I think it's overly generous to extrapolate for five years into the future the 32% by which the latest quarterly revenue is above the same year-ago quarter: Facebook's revenue over the last two quarters has been largely flat. In fact, if I were to base my extrapolation on just the last two quarters, I would find that even my original fair value estimate of $13.80 per share is too high.

The bottom line?

I can find nothing in Facebook's latest earnings report to suggest that a fair-value estimate for the stock of $13.80 needs major revision. No wonder Facebook shares were down so much in the wake of that report.