After Twitter spent its formative years just trying to stay operational, the San Francisco-based microblogging service seems to be entering a new creative period. Twitter wants you to make more friends — but the fast-growing startup also seems to be finding smarter ways to cultivate allies.

The site’s developers have spent the last week trying out a number of new features on the homepage and users’ profile pages, in addition to debuting an official Tweet button to make it easy to share Twitter posts on other websites.

Twitter hasn’t entirely escaped its unstable past: Its Tweet button accidentally made websites that had implemented it inaccessible shortly after its rollout.

Perhaps to avoid such debacles, Twitter is rolling out new features progressively to small groups of users, a standard practice in the Web industry — but that means not everyone will see them when logged in. But together, they paint a picture of Twitter seeking to get users more engaged with the service.

Most users earlier in the week noticed a ‘Who to follow’ section on their homepage. Taken right out of Facebook’s popular ‘People You May Know’ which suggests friends on the social network, the feature assembles a list of users similar to those you already follow.

Similarly, some also reported seeing ‘You both follow’ on other users’ profiles, and its inverse, ‘Also followed by’. Facebook has long listed ‘Mutual friends’ on its profile pages. It’s a handy way to compare how much you have in common with other users in terms of the people you follow, while the latter is a good way to check for validation of a user from those you follow. Both can be useful indicators when considering whether to follow a particular user.

Kris Colvin from FreshID reported seeing a section called ‘Included in’ on her Twitter profile. Colvin said it appeared to be based on data from Tlists, a site that crawls Twitter lists to identify authoritative users on Twitter and categorize them. It then creates new lists to categorize that user. For example, Colvin’s Twitter profile shows “Included in: Advertising & Marketing, Social Media, Kansas City.” The categories link to Tlists lists.

That Twitter would integrate its site so closely with a third-party service is surprising. In the past, Twitter has acquired startups developing useful Twitter-based services, like the Summize search engine, or simply imitated their work.

Fred Wilson, a venture capitalist who invested in Twitter, warned third-party developers who were merely “filling holes” that Twitter would soon grow to take their place. In May, Twitter bought Tweetie and rebranded the app as its own official offering for the iPhone, while simultaneously also launching its own BlackBerry app to displace market leader UberTwitter. In the same month, the company also revealed its ‘Promoted Tweets’ ad model, putting weight on similar sponsored tweet offerings like Ad.ly and TweetUp, and later condemned their existence by barring third-party ads out of user timelines in APIs.

The Tlists integration suggests a new, more cooperative approach, as CNET’s Caroline McCarthy noted about Twitter’s new Tweet button, which was developed with help from third-party sharing service TweetMeme.

With Twitter pushing for ubiquity and control over its branding and reach, the challenge for the company will be to carry out its interests while tapping its active and engaged developer base. It may have well discovered that the fastest way to fill a hole is to hand a friend a shovel.