In cleantech, as in most realms of emerging technology, venture capital acts as a sort of incubator for the youngest, most promising technologies. That's why it's a cause for concern when venture capital investment slows or shrinks.

That's just what happened last year. In 2011, venture capital investment in early-stage cleantech companies fell by 4.5 percent, to $4.9 billion, compared with the 2010 tally, according to a round-up of full-year data by Ernst & Young published Feb. 1, based on data from Dow Jones VentureSource.

Whether this downtick is cause for concern is open to argument. The question links to hot-button issues being debated in Congress, on the campaign trail, and in the media. I, for one, believe that given the headwinds facing cleantech, the numbers are cause for optimism. They're good news, but I wish they were better.

To make my half-full case, note that cleantech venture capital investment has been resilient despite both economic and political headwinds. Last year's funding remains 29 percent higher than its 2009 total, when overall venture flows crashed in the wake of the global financial crisis.

What's more, cleantech is nurtured by other streams of capital. As I reported last month, global investment in mature renewable energy technologies -- new wind farms, solar panels, and the like -- expanded by 5 percent, to $260 billion last year. That rise helped put total investment in renewable energy, efficiency, smart grid and related technologies over the trillion dollar mark last year.

Still, I'm a worrier. And there are reasons to furrow my brow at these numbers.

However promising cleantech may be, venture capitalists are finding more alluring options in other sectors. Cleantech's decline comes despite a 10 percent rise of overall venture capital investment. Globally, for the year, investors placed $32.6 billion into 3,209 venture deals, according to Dow Jones Venture Source.

So while cleantech retreated, investment in healthcare and IT startups remained roughly steady. The big winner? Consumer information services -- think Twitter, LivingSocial and Zynga -- pulled in $5.2 billion, up 23 percent from the prior year.

But before I complain any further that clean technology shouldn’t be losing out to Twitter, let alone Facebook, here's a bit more on what went down in cleantech over the past year.

• Battery technology is hot. Energy storage continues to attract interest, and growing flows of money. Venture investment in batteries rocketed up by 253 percent. And this is bound to accelerate. Growing volumes of electric vehicles, plus the graduation of wind and solar from emerging-tech status to mature technology, are all driving demand for energy storage, in a dizzying array of niches.

And while some segments of battery manufacturing are mature -- increasingly subject to the sorts of commodity price dynamics driving down prices of solar PV -- there is arguably bigger potential for scientific discovery to upend today's batteries.

• Investment is tilting towards more mature plays. Cleantech companies already generating revenue garnered 69 percent of the funding, up from 50 percent in 2010.

• M&A exits dominate. Given the parlous state of IPO offerings, mergers & acquisitions continue to be the main path to maturity for cleantech players. In 2011, a total of $2.9 billion in M&A deals involved cleantech startups, some 79 deals, according to Ernst & Young's analysis.

• IPO drought lingers. Just five companies IPO'd in 2011, not many more than the three that listed a year prior. Biofuels dominated last year's public debuts, with Solazyme, Gevo, and KiOR. Intermolecular, a semiconductor R&D company focused on cleantech listed in the final quarter, as did Rentech, a clean energy solutions provider. The five raised a total of $688 million.

Next page: What Facebook's IPO means for cleantech