On the heels of last week’s less-than-stellar quarterly earnings report, there’s more bad news for Blackberry and RIM today. Seesmic has just announced that, effective June 30, it will no longer support its popular Twitter client on Blackberry.

The words of Seesmic’s announcement make the company’s rationale clear: the company is discontinuing support for Blackberry “in order to focus development efforts on our most popular mobile platforms: Android, iOS, and WIndows Phone 7.”

In last week’s earnings report, RIM revealed that its Blackberry sales were not as strong as it had anticipated. But sales of devices are only part of what makes (or breaks) a smartphone nowadays. The other piece of the puzzle is the vitality of the developer ecosystem, particularly as customers have more choices for mobile phones, the availability of their favorite apps is a major selling feature. No doubt, losing developers working on the Blackberry platform will only make matters worse.

Of course, this may just be one high-profile departure from that ecosystem, but there seems to be a sense that the RIM ship is sinking. At the same time, some analysts are predicting that Windows Phone 7 may see a large jump in adoption, and so focusing on that platform, along with Android and iOS would make sense.

The recommendations for Seesmic for Blackberry users, according to the company: “We encourage those effected by this change to try out Seesmic for Android, iOS, and Windows Phone 7.” In other words, buy a new smartphone. Ouch.