Last week brought two major announcements from Twitter. On Thursday, the company announced an official application for BlackBerry. On Friday, Twitter announced that it had purchased Atebits, the makers of the iPhone app Tweetie. Over the weekend, there was substantial discussion and a fair amount of hand-wringing by third-party developers, many expressing their frustrations about the company’s direction. Attempting to reassure developers in advance of next week’s Chirp conference, Twitter API lead Ryan Sarver responded by email to some of these concerns.

Certainly Twitter isn’t the only company at the center of debates about control of a platform (Apple, Google, and Microsoft come to mind), but in light of the flurry of responses to Twitter’s moves, it is worth considering some of the (perhaps contradictory) lessons for startups that can be gleaned from the past week’s events.

Find your niche: Much of the third-party development on Twitter has served to address gaps in the original product: mobile clients, URL shorteners, photo sharing, and search for example. As VC and Twitter investor Fred Wilson argued in a blog post early last week that tipped the hand, perhaps, to where Twitter was headed, there is still room for the development of “killer apps” in social gaming, enterprise, and analytics.

Innovate and adapt: Find your niche, but then be prepared to innovate and adapt. Some have suggested that Twitter’s acquisition of Tweetie might not bode well for other Twitter clients like Seesmic and Tweetdeck, unless the two can continue to innovate. By adding new features unavailable via the Twitter website, and by linking streams from Facebook and LinkedIn, they have established themselves as more than just a Twitter client – but the pressure is certainly on for these to continue to distinguish themselves from the official Twitter applications. “Of course we’re hole fillers,” Seesmic founder Loic Le Meur admits, explaining that while that’s a good place to start, it isn’t the right place to end.

Look beyond the platform: As Mark Suster writes of both Twitter and the iPhone, it is important to think beyond the platform, contending that startups should not think of Twitter “as a business but rather as a channel.” In other words, a platform like Twitter should be a used as a way to reach customers but, unless you’re Twitter, should not be the vehicle itself.

If this is the “inflection point” for Twitter, the tasks for startups will be to learn the lessons from this critical juncture in the platform’s history, balancing the sometimes contradictory needs for specificity and flexibility and innovation and stability.