How can Twitter make money? It’s the one question everyone has been asking and the one question Twitter hasn’t been in a rush to answer.

But as Twitter continues to experience massive growth during a painful recession, revenue is a subject that Twitter probably shouldn’t put off for too long.

There’s a new sign that this may indeed be the case: ads have been spotted in the wild on Twitter.

Right now, the small ads that appear under a profile’s vital stats are only promoting Twitter features but everyone who has seen them recognizes the potential: it wouldn’t be hard for Twitter to sell these.

Obviously, they’re simple. So was AdWords. Could these little ads do for Twitter what AdWords did for Google?

Probably not. The one thing AdWords had going for it that Twitter probably doesn’t – pure intent – is crucial. Nevertheless, I could see lots of people purchasing these ads to promote themselves and their businesses.

If Twitter offered general targeting options based on demographic information and interests (based on an analysis of the content of a user’s tweets), it just might have something really big, even if it isn’t AdWords big. I know I’d probably buy some ads of my own just as an experiment.

Hopefully Twitter gives me and you that opportunity.

But Twitter also has to be careful. It doesn’t want to squander its opportunity. If Twitter is to succeed financially, it can’t become Facebook, which is plagued by low-quality ads promoting everything from diet programs to shady dating sites.

Quality, relevancy and decency should be top of mind as Twitter plans out an ad model. Given the time it has taken for Twitter to start experimenting, I’d like to believe it has been thinking about these things.