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Seesmic, a popular third-party Twitter client for desktop and mobile devices, announced Monday it would end support for BlackBerry handsets at the end of June. In a blog post, the company shared the news, saying the reason for the decision is “in order to focus development efforts on our most popular mobile platforms: Android, iOS, and Windows Phone 7.” One developer certainly doesn’t make a trend, but given RIM’s (s rimm) slowing sales growth and lengthy transition to a new mobile smartphone platform, this could be the beginning of a terrible trend for BlackBerry devices.

Based on a literal interpretation of Seesmic’s reasoning, it’s possible the application simply never became as popular on BlackBerry handsets as it is on those running other platforms. A quick check of Seesmic on the BlackBerry App World website shows a total of 516 reviews, with the average rating of 2.5 stars out of 5. But conventional wisdom dictates a developer would want to improve upon the app, not totally abandon it; unless, that is, the platform is too limiting, there isn’t much potential upside to making improvements, or both.

We’ve written before about which will be the third major mobile platform behind Google Android (s goog) and Apple iOS (s aapl). The jury is still out, and while there’s room for multiple mobile operating systems, there will be dominant ones that benefit from application lock-in. It’s clear that Seesmic is betting on Microsoft’s Windows Phone 7 (s msft) system based on this move. So too are major research firms: In May, Gartner (s it) predicted Microsoft will surpass Apple for the no. 2 smartphone OS spot by 2015. Last week, IDC joined the bandwagon and said the same, mainly because of Microsoft’s new partnership with Nokia (s nok).

We won’t know for some time if such projections are accurate, but we — and the folks at Seesmic — do have access to some current worrisome statistics related to RIM. The company reported lower than expected earnings last week (after it already lowered expectations previously) and suggested a challenging year ahead. RIM introduced a new and modern operating system in QNX, used for its PlayBook, and will be transitioning its smartphone lineup to the new platform in the future. But RIM hasn’t provided a clear timetable on such a move, and its latest BlackBerry handsets appear to be delayed.

At this point, the BlackBerry operating system looks to be a legacy platform that will be a succeeded by an incompatible new platform at some unspecified future time. Device sales aren’t growing as fast as the general smartphone market, and even the company’s new tablet sales are under 500,000 total units after two months on the market. And even that number only represents the number of tablets shipped, not those sold. Again, one third-party developer dropping a platform doesn’t make a trend, but it could beginning of one, in light of the overall challenges RIM is now facing.