NEW YORK (CNNMoney) -- Here's a sign of just how far BlackBerry's star has fallen: The maker of one popular app is pulling the plug, citing a "steady exodus" of users gravitating instead to Android and the iPhone.
Visual voicemail company YouMail is ending development on its BlackBerry app. YouMail CEO Alex Quilici said in a blog post that the decision was "bittersweet," because BlackBerry "got us our first million registered users and put us on the map as a company."
YouMail's app is currently among the top-10 most popular apps in the "productivity" category of BlackBerry's App World store. But Quilici says the customer base has collapsed.
As he wrote on YouMail's blog: "On many days we're now getting fewer BB users than Windows Phone 7 users, and we don't even have a Windows Phone 7 app!" (YouMail's apps for Windows are developed by third parties that license YouMail's API.)
"When we began to see that trend, it definitely made us think," Quilici told CNNMoney.
YouMail, based in Irvine, Calif., is growing rapidly on Apple's (AAPL, Fortune 500) iPhone and Google's (GOOG, Fortune 500) Android platform, Quilici said. Almost 2.5 million people are currently registered app users.
"Our company is only 10 people in total, and they're all development or customer support," Quilici said. "We need to focus on where we see growth. And right now, it's not BlackBerry."
It's a long way from the former YouMail-BlackBerry love affair. YouMail started out as a Web-based service in 2007, then added mobile in 2009 after BlackBerry users requested a handset app. Research in Motion (RIMM) gave YouMail a lot of support, Quilici said, by highlighting it several times as a featured app in App World.
But the customer base started dwindling, and development was taking exponentially longer for BlackBerry than for iPhone or Android.
"A feature that took two to three days to build on Android and the iPhone took two to three weeks on BlackBerry," Quilici said. "The platform is getting long in the tooth."
RIM did not respond to a request for comment. Quilici, who reached out to RIM on Monday after news articles began appearing about YouMail discontinuing its BlackBerry support, said he also hasn't heard back.
One developer leaving obviously isn't a death blow. But it's an ominious sign for RIM, which is struggling to stay relevant in an industry it once dominated.
If BlackBerry can turn itself around -- or if RIM was willing to foot the bill for app development, a tactic Microsoft is using to seed its Windows Phone app store -- Quilici said YouMail would gladly come back to the platform.
"We're all rooting for BlackBerry over here," he said. "IPhone was a success because it was different and cool, while Android got attention for being largely free and open. I hope RIM finds something like that to differentiate itself." 
