In an eye-rubbing moment, Bloomberg BusinessWeek is reporting that Facebook's valuation is up to a whopping $41 billion.

The reporter's source is an unnamed one with access to transactions on SecondMarket.com, a website where stock-owning employees at privately held upstarts can sell their shares to third-party buyers.

Even if the account is true, it obviously doesn't mean that Facebook is worth that much. It would make the social networking site the country's second most valuable dot-com that relies primarily on ad revenue. Only Google (Nasdaq: GOOG) would be more valuable.

Facebook is rocking, but it leans primarily on ho-hum display advertising. Registration growth is also decelerating on a percentage basis. It's not that $41 billion isn't possible. It just doesn't seem sustainable beyond a single second-market equity swap.

However, if Facebook is really worth more than $40 billion, it would only force me to shake my head harder when I think about what United Online's (Nasdaq: UNTD) Classmates.com could have been.

Classmates dismissed
Stories of near misses with Facebook-esque fame are plentiful.

"We can't always assume that MySpace will be the top dog," I wrote about News Corp.'s (NYSE: NWS) then-happening hub four years ago. "After all, pioneers like Friendster and Tribe.net once ruled the social networking roost."

MySpace, Friendster, and Tribe could have been today's Facebook, but Classmates was around even before then.

Unfortunately for Classmates.com, the site decided to stick to a premium model. It figured that folks would shell out good money for subscription access to their former high school and college buddies. It never dawned on Classmates, it seems, that the better model for viral growth would be to tear down the pay wall and let the billable ad space on their page views bring home the bacon.

United Online acquired Classmates.com for $100 million in 2004, just as Mark Zuckerberg was coding magic in his Harvard dorm room. It tried to go public three years later -- when Facebook was turning heads after a Microsoft (Nasdaq: MSFT) investment valued Zuckerberg's young company at a whopping $15 billion -- but it had to scrap the IPO.

Despite having 2.7 million paying subscribers, Classmates had a long trail of operating losses. With free social networks growing at breakneck speeds, it was hard to round up interest in a site that was unfashionably early to the nostalgic appeal of alum interactions and losing money despite the stiff cover charge.

This classroom's a mess
A year ago, I got an email from Classmates. A high school pal that I hadn't heard from in ages had signed my virtual guestbook. I didn't have access to it, since I wasn't a premium member. I never had been. Like many users, I registered on the site ages ago, laughed at the tollbooths, and drove the other way.

This week, I was sent another promotional pitch, offering me free email exchanges for a limited time because this marked 25 years since my high school graduation. I decided to take this opportunity to see if Classmates had learned the Web 2.0 lessons that it had flunked out on earlier.

Frustrated at what I was able to do on the site with my limited access, I bit on a seven-day free trial offer. I know how these bait-and-switch schemes go, so I canceled the moment I was done sniffing around.

What a joke. Able to finally access the 2009 "signature," it turns out that there was no message. It was just an old chum who had checked out my profile. I decided to check out the message board for my high school and university. Both were dead. I was only the second person to visit either forum this month, and there were threads dating as far back as 2003 among the most recent posts.

I was teased about digitized yearbooks, so I checked it out. Unfortunately I'm not from my high school's class of 1976 -- the one and only book on the site.

People actually pay for this?

Pay the right ransoms
There's nothing wrong with premium models that offer premium experiences. Folks pay Ancestry.com (Nasdaq: ACOM) an average of $18 a month because the leading genealogy site has billions of digitized records and a lively roots-digging community. Drivers pay Sirius XM Radio (Nasdaq: SIRI) for access to commercial-free music and dozens of proprietary talk channels.

How well would Ancestry.com be doing if its product weren't as good as free family-tree builder Geni.com? Would Sirius XM be able to keep its satellites going if it were offering programming that was inferior to terrestrial radio?

Classmates is a website that relies on the free flow of information, yet the pay wall and protective contact tactics keep it from being the viral winner that it could have been.

United we strand
United Online commands a market cap of just over $0.5 billion. It's about far more than Classmates.com. Its flagship business remains low-cost online access through NetZero and Juno. It also runs florist exchange FTD.com and loyalty rewards site MyPoints.com.

Things could have been so different if United Online had gone for the mainstream jugular with Classmates.com. It already had a healthy head start over Facebook when it was acquired, but United Online stuck to the premium model, when the model itself dictates an inferior social network.

If it were Classmates backed by a $41 billion valuation -- and not Facebook -- United Online shareholders would be sitting on an 80-bagger right now.

It's not as if United Online didn't have a clue. Its NetZero service turned heads initially because its name actually stood for a free, ad-supported ISP. It offers a limited form of that now, but ad-supported user-generated content is a lot easier to bankroll than a free access provider.

Maybe it's not too late, but if United Online ignored my plea to tear down the wall three years ago -- when Facebook was still vulnerable and in the crib -- I'm sure the company and its poorer shareholders aren't hearing me now.

Do you own a company that just missed out on something huge? Share your thoughts in the comment box below.

Google and Microsoft are Motley Fool Inside Value selections. Ancestry.com and Google are Motley Fool Rule Breakers recommendations. Motley Fool Options has recommended a diagonal call position on Microsoft. The Fool owns shares of Google, and Microsoft. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors.

Longtime Fool contributor Rick Munarriz remembers when social networks were an offline endeavor. He does not own shares in any of the companies in this story. He is also a member of the Rule Breakers analytical team, seeking out the next great growth stock early in its defiance. The Fool has a disclosure policy.