Probably the most important lesson we learned from the Internet bubble of 1999-2001 (other than the fact that Pets.com was a terrible idea) was that rapidly rising revenue is meaningless if there's little hope of a profit on the horizon. The majority of dot-com companies went belly-up once investors realized there was nothing to sustain already-inflated prices.

Fast-forward to 2011, and we are arguably on the precipice of another potential bubble -- this time from social media and newer technologies. Unlike 1999, many of these companies at least have revenue and have some idea of when they expect to be profitable. Still, some recent companies are exhibiting plenty of growth, but I'm extremely skeptical as to whether they will ever turn a profit. Let's take a look at two potentially troubling candidates.

Pandora Media (NYSE: P)
Pandora Media is all the rage because it has a rapidly expanding subscriber base and presents the only true challenge to Sirius XM's (Nasdaq: SIRI) music dominance. Despite my long-running denial that Sirius XM would ever be profitable, the company now is, and it's now Pandora that looks like it may wind up the perpetual loser.

Pandora Media isn't lacking revenue or subscriber growth, logging a 136% jump in revenue and a near doubling of subscribers from 18 million to 34 million in its most recent quarter. The company generated 86% of its revenue from advertising with the remaining 14% coming from ad-free premium subscriptions. But more importantly, losses are increasing -- a trend that has been prevalent since Pandora was founded 10 years ago.

Working against Pandora are the steadily increasing royalties from the music industry and the higher costs associated with being a publicly listed company. Currently, Pandora is paying out approximately half of its revenue in the form of royalties, and that trend is likely to continue. It's very unlikely that Pandora will be able to break itself free from rising royalty costs, and even if it can, legal fees will pick up where the royalty fees left off. I don't see how Pandora will ever turn a profit.

Tesla Motors (Nasdaq: TSLA)
I'm no dummy; I know this is going to be an extremely unpopular opinion, especially with the Model S electric vehicle due out by mid-2012, but I just can't see Tesla turning a profit anytime soon -- if ever!

First of all, Tesla is facing some major competition in the form of Nissan's Leaf, Ford's (NYSE: F) electric Focus, General Motors' (NYSE: GM) hybrid Volt, and Honda (NYSE: HMC) entering the electric car free-for-all. Just because Tesla was one of the first car companies capable of producing electric vehicles at a rapid rate doesn't guarantee the company success.

Current analyst projections call for Tesla to grow revenue by threefold in 2012 with the introduction of the Model S, but call me not so convinced. On countless occasions in recent history, Tesla has run into parts supply delays and technical snafus. Let's just say I wouldn't be shocked to see the debut of the Model S pushed back beyond the mid-2012 date the company provided.

Then there's the company's debt, which will begin to be addressed in 2013. Based on loans from the Department of Energy in January 2010, Tesla will need to begin making quarterly payments on its debt in 2013 that should amount to roughly $12 million per quarter. That may not sound like much, but these interest payments could be enough to keep the company unprofitable even if the company is able to sell 20,000 EVs each year.

Foolish roundup
Pandora and Tesla boast market values of $2.8 billion and $2.9 billion, respectively, and if history repeats itself these two companies may find their bubbles bursting sooner than later. While I don't have a crystal ball, I do have plenty of facts and history to stand behind these claims.

What's your take? Share your thoughts in the comments section below and consider adding Pandora Media and Tesla Motors to your watchlist to see if my prediction holds water.

Fool contributor Sean Williams has no material interest in any companies mentioned in this article. You can follow him on CAPS under the screen name TMFUltraLong. The Motley Fool owns shares of Ford. Motley Fool newsletter services have recommended buying shares of Ford and General Motors. 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. The Motley Fool has a disclosure policy that'll never go belly up on you.