It's a wonderful feeling to be in the right place at the right time. That's where SecondMarket and SharesPost found themselves during Facebook's private years.

The two companies are online investment markets where those who hold stock in non-public companies can sell it to wealthy individuals and institutions. And both did well selling shares owned by early Facebook investors and employees who wanted some liquidity—particularly as share prices traded in the $40 to $45 range and the marketplaces got cuts of the sales.

According to one estimate, a third of SecondMarket's revenue last year came from trading Facebook shares. But clearly those days are over, now that the social network giant has gone public (with share prices well off the $38 IPO price). Now both face a problem common to rapidly growing businesses: After you've ridden the first zeitgeist, where you do find the next one?

The Next Crop of Hot Start-ups

It's not that no other shares will trade. Go to SharesPost, for example, to see the list of companies. Check for those with offers to buy or sell and the 149 companies drop to 18. There are some notable names left: Square, Pinterest, Gilt Group, and Etsy, to mention a few. SecondMarket has thousands of users watching for shares of Twitter, Dropbox, and Foursquare, among others.

All these companies have promise, particularly as employees look to cash in at least some of their holdings sooner rather than later. And the Facebook experience suggests that when the amount of stock available to trade is slim, investors might pay more than they would in an IPO.

But that still probably doesn't replace the business that Facebook supplied. (If it could, Facebook's reported share of SecondMarket's revenue would likely have been much lower.) SharesPost lists "recent" offerings, but the newest were $15 million in Facebook stock and $10 million in TrueCar shares back in December 2011. Additionally, some companies don't want shares traded on private markets, and are prohibiting employees from doing so, as Facebook eventually did.

Looking Beyond Start-ups

So the companies are doing what any entrepreneur would in the circumstances: trying to expand into additional markets. SecondMarket already trades in community bank stock, fixed income securities, and bankruptcy claims, for example, and plans to expand into such areas as wine and distressed residential loans.

It might seem a bit of a stretch, but remember that the only people to whom either company can legally sell are accredited investors. The law defines accredited investors as those with a net worth of $1 million, excluding a primary residence, or an income of $200,000 or more ($300,000 or more with a spouse) in each of the last two years with the likelihood that the income will remain the same or increase in the current year. 

One possible partial savior for both companies could be the recent JOBS Act, which makes it easier for smaller companies to seek investment, often through such platforms as SecondMarket and SharesPost.