When Facebook (NASDAQ:FB) shares soared 30% the day after the social network reported its blowout earnings I couldn't help wishing I had emulated my Facebook CAPScall in my real-money portfolio. Too late. Or is it? Is there still upside left to Facebook's stock?

"Facebook Premium"
Pandora One subscribers grew 114%, year over year, in the company's most recent quarter. In fact, the ad-free version of its streaming music was the company's fastest growing revenue stream during the quarter.

What does this have to do with Facebook? According to Twitter co-founder Biz Stone, giving members an option to pay for an ad-free Facebook experience along with a few other exclusive benefits could generate an additional $12 billion in revenue, annually.

Pandora has Pandora One for $36 per year. Flickr has an Ad Free account for $50 per year and a Doublr account (ad free plus 2 terabytes of photo and video space) for $500 per year. Facebook has ... well, on Facebook you must endure advertisements whether you like it or not. For now.

Was Biz Stone right?

Greenlight decided to put Stone's assumptions to the test, polling 500 Facebook users. The study revealed several interesting tidbits.

As it turns out, not every Facebook user appreciates ads. Surprise, right? Greenlight's survey showed that about 70% of respondents never or rarely click on ads or sponsored listings. Greenlight CEO suggests this means that this is "indicative that consumer apathy [for advertisements on Facebook] is very real."

Just how many members would pay for an ad-free experience?

Source: Facebook Newsroom.

Fifteen percent. In fact, 8% indicated that they would spend $5 or more per month.

If 15% of Facebook's 1.15 billion members paid $7.50 per month for an ad-free experience, Facebook could earn an additional $1.25 billion every month. That would have essentially tripled Facebook's second-quarter revenue.

Why not?
If some members would love to pay for an ad-free service, Facebook should let them. With revenue like this, Facebook could spend more time on improving the user experience and less time spicing up its ad business.

This is just one way Facebook could increase its revenue. I recently detailed a way Instagram could contribute to the top line, too. Certainly there are even more ways Facebook could monetize its lockdown on more than 1 billion of the earth's population.

During Facebook's first year as a public company, I missed the mark. Facebook doesn't need to grow into its valuation with existing streams of revenue. It just needs to introduce new streams. Easier said than done, but still a realistic development.

I may have missed the party, but I still might buy Facebook shares.