Neither of those parties have the potential to become a real mass party capable of challenging either the Democrats or the Republicans. Any potential mass third party would have to have a stable group of supporters that compose a non-negligible portion of the population (which rules out the upper middle class Libertarians and Greens entirely), as well as a stable base for funding and party outreach and development.
To add to what TNF said, the American system is naturally a two party one. Unless one of those parties manages to displace the GOP/Dems (extremely unlikely), they won't have any long term success.
Except for the presidential aspect - which I agree is a very important one - the US system is not all that different from the UK one though. You're both electing your lower house representatives from one-man districts in a winner-takes-all manner. In the UK this has developed from a traditional two party system (Labour & Conservatives) into a three party system which we've had for a while now (with the Liberal Democrats) to what seems to becoming right now a four party system (adding the UK Independence Party as well). Additionally to this, you have the regional, nationalist parties of course of Scotland, Wales and Northern Ireland. Now, since the US is just one nation, and the only thing that can challenge that is the influx of immigrants (since the Native Americans have never had any political power or hardly even influence), it doesn't seem likely to turn into several nations in the future either. Now, there's no doubt that the entire Western Europe is a much more modern place than the current USA. Especially when it comes to religiosity, that's the case. The only Western European nations I can think of that somewhat resembles the power religion has in US politics and everyday society would be Italy and Malta, and I think even there the religious aspect is rapidly waning. Especially now that Berlusconi seems to have lost his political influence altogether and Italians seem to finally be ready to move on and face modernity once and for all (although it's all very complicated with the Vatican State located in Rome).
The US will eventually match up with where Western Europe is at present, the question is simply how long it will take (the South will be the real drag on this timeline of course). At best, the US remains about 10 years behind Western European society when it comes to modernity and embracing academic approaches to things rather than simply trusting your gut feeling or what your spirituality tells you is the right thing. At worst, the US could be as far behind as 20 to 25 years. I remain optimistic, especially in an ever changing, rapidly accelerating globalized world, and would say that the US electorate probably only will lag Western European societies with about 10 to 15 years.
Which could mean that the US could have at least one strong third party come 2030, simply because much of the electorate will be truely fed up with the status quo and the ultra rigid parties they can chose between at present (honestly only the Republican party is ultra rigid, but that's another debate). To pretend that the US is doomed and destined to remain with a two party system for ever and always is simply to indulge with superstition. People used to say that most European democracies would remain with only two or three major parties forever as well, but look at what has happened to basically every single Western European nation for the past few years and decades. Spain has changed from a two party system to a multi party system because voters have been fed up with the two major parties for quite some time. In Greece the same has happened thanks to the everlasting damage caused by the financial crisis. In the UK it is under way as well. In France, the two major parties have lost almost all credibility. Even in Germany, minor parties are starting to gain more ground, despite Angela Merkel's astronomical popularity. In Sweden the multiplication has gone on for a long while already. Basically I don't think there's a single exception to the rule as far as I can see. Voters simply want diversity. Just like consumers demand competition in the private service sector and multitude of products to chose from in their local supermarket, more and more this happens to be the case even in the public arena of politics. At least that's what I believe.
Changes sometimes take a long, long time, other times they happen over night (like they've done in Greece and in UK with their Independence Party and in the US when it comes to gay marriage and cannabis legalisation). It's hard to predict exactly when the US will get a major third party, because history always surprises us to some degree, but I'd say it's quite likely to happen at some point during the 21st century. My prediction is that it will happen as some sort of local revolt first. Perhaps The Green Party will first gain strenght in Vermont or Maine. Perhaps the Libertarians will first break through in Colorado or Nevada. Immediately it will cause a tsunami of media attention, which in itself will accelerate the changes by several years, perhaps even decades. Modern history shows us that rapid changes are more the conventional wisdom than the exception (see the rapid rise of the Tea Party for instance). Call it the Meme of Politics or the Facebook Revolution of Politics if you want.
