Twitter is launching new efforts to boost usage by suggesting location-centric tweets and improving how it recommends whom you should follow. The moves come as a new analysis shows that the number of Twitter users soared to about 75 million in December 2009–from fewer than five million one year earlier–but that only about 15 million are regular users.

The analysis, by RJMetrics, a business analytics firm in Camden, NJ, is posted here. It opens a window on Twitter’s otherwise closely held usage data. “While [15 million] seems like a small percentage, it’s a tremendously large number on an absolute basis. Twitter’s value comes from those 15 million passionate, active users,” says Robert Moore, CEO of the analytics firm.

Twitter members issue 140-character-maximum messages called “tweets” and select whose tweets to follow. Moore says that his analysis only captures how many tweets Twitter users create, and how many people they follow. It does not measure how many tweets each user actually reads. “If we were able to monitor that usage, we might find that the ‘active user’ base is even larger,” he adds.

Twitter is now moving to give users new ways to sift through and find useful tweets. It has long offered users a list of real-time “trending topics” to explore–essentially all tweets containing the most-popular terms, such as “Haiti” or “Super Bowl.” Now it is beginning to offer a refined version called “local trends”–lists of tweets bearing popular keywords grouped by region. Initially, however, these regional groups consist only of six countries (the United States, Canada, Brazil, Ireland, Mexico, and the U.K.) and 15 cities (13 U.S. cities plus Sao Paulo and London). The feature should be available to all users in those areas by the end of this week.

Right now the existing trending topics are often far from relevant, and some of the tweets that appear when you click on a topic amount to spam. Yesterday morning, for example, a check of trending topics (done in Boston, for which “local trends” aren’t yet available) showed that two of the top trending topics were “#perfectbody” and “#idothat2,” which led to trivial tweets, some containing links to porn sites.

Twitter did not reply to requests for comment yesterday. But Evan Williams, one of Twitter’s founders, alluded to the coming wave of location-based Twitter services in a blog post published last December announcing the acquisition of a company called Mixer Labs. This company created GeoAPI, an application that helps developers write code to make effective use of GPS data, and that includes a location-based database of 16 million businesses and tens of thousands of tourist sites and other points of interest.

The new Twitter feature depends, of course, on knowing the accurate location of a Twitterer. Many mobile devices allow GPS data to be affixed to a tweet or other communication, and Twitter can accept such data. But the mobile phone user must specifically authorize the GPS transmission for privacy reasons. So right now, for the most part, Twitter determines the location by noting a Twitterer’s Internet protocol address–a rough proxy for geographical location–or the home city she listed when she signed up.

Either way, “when current location is added to tweets, new and valuable services emerge–everything from breaking news to finding friends or local businesses can be dramatically enhanced,” Williams wrote in the December blog post. “Our efforts in this area have just begun.”

Google and Bing and smaller search engines such as Cuil have all recently inked deals with Twitter to obtain streams of its data for real-time search. Dylan Casey, product manager for Google real-time search, said in a recent interview that adding location data would be crucial to improving what kinds of tweets Google supplies to its searchers. “We are heavily focusing on the mobile experience, and adding even more and more precision to the comments that you are already getting,” Casey says. If you tweet that you’ve just seen a major traffic accident, for example, it’s helpful to know the exact stretch of roadway–something GPS coordinates can provide.

In a separate effort, Twitter is working to improve its system for recommending what Twitterers to follow. It has always suggested prominent Twitterers, but it recently launched a new suggestions page fueled by algorithms that identify clusters of people with similar interests for you to choose from.

“Rather than suggesting a random set of 20 users for a new user to follow, now we let users browse into the areas they are interested in and choose who they want to follow from these lists,” wrote Josh Elman, a Twitter product manager who recently defected from Facebook, in a blog post published on Thursday. “These lists will be refreshed frequently as the algorithms identify new users who should be suggested in these lists, and some that are not as engaging to new users will be removed.”