After four years of operating in the open, letting users and developers experiment relentlessly, Twitter is locking some of its doors.

"It is critical that the core experience of real-time introductions and information is protected for the user and with an eye toward long-term success for all advertisers, users and the Twitter ecosystem. For this reason, aside from Promoted Tweets, we will not allow any third party to inject paid tweets into a timeline on any service that leverages the Twitter API," Chief Operating Officer Dick Costolo wrote in a blog post yesterday. [Emphasis added.]

In years past, I've likened Twitter to Google (Nasdaq: GOOG), because of how it indexes the real-time zeitgeist that exists on the Web. The free-flowing open-source nature of Twitter seemed, for lack of a better word, Googly.

Now, looking at Costolo's language, I see more in common with Apple (Nasdaq: AAPL), which is forcing iPhone developers to use its tools and write only to the programming interfaces it deems appropriate. Adobe (Nasdaq: ADBE) has all but abandoned efforts to get Flash on the iPhone as a result.

Yet I like what Twitter's doing here. First off, it's nowhere near as draconian as what Apple has planned. As Costolo points out in his post, this isn't a ban on Twitter advertising. Instead, the social superstar is taking steps to ensure that tweet-streams aren't unnecessarily polluted with content no one wants. Twitter, as gatekeeper, will keep the polluters out.

"Third party ad networks are not necessarily looking to preserve the unique user experience Twitter has created," Costolo wrote. "They may optimize for either market share or short-term revenue at the expense of the long-term health of the Twitter platform."

Put another way: If Twitter's $1 billion valuation is directly related to the intelligence provided by those inhabiting the network, anything that makes Twitter dumber is dangerous. Unrelated ads could have that sort of effect.

So Twitter is treading carefully, as it should. As much as Promoted Tweets might help more companies like (Nasdaq: AMZN) and Dell (Nasdaq: DELL) use the service to sell, spammers have a history of ruining great platforms. (Email, for example.) Limiting API access reduces the chances they'll do the same to Twitter.

But there's also a risk in taking this step. Twitter's success owes largely to third-party contributors who've built upon the API. Locking doors could leave willing innovators out in the cold at a moment of maximum opportunity -- one in which tectonic shifts in the advertising industry are creating superlative-sized opportunities.

Costolo and the entire Twitter team need to strike the right balance. If they do, Twitter's resemblance to Apple will reflect its performance more than its policies.

Would you allow Twitter to advertise in your tweet-stream? Discuss in the comments box below.

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Fool contributor Tim Beyers is a member of the Rule Breakers stock-picking team. He had stock and options positions in Apple and a stock position in Google at the time of publication. Check out Tim's portfolio holdings and Foolish writings, or connect with him on Twitter as @milehighfool. The Motley Fool is also on Twitter as @TheMotleyFool. The Fool's disclosure policy needs a glass of water. Would you mind?