In fact, if the publicly available download stats in Google play are accurate then Aldiko has actually been downloaded more times than the Kobo or Nook Android apps (I kid you not). Of course, we don't know how many apps are actually in use, but at this point there's little reason to think that Aldiko is used less on Android than its brand name competition.
The Aldiko app was initially launched in June 2009. At that time the developers split their efforts between developing their app and creating standalone ebook apps for publishers. O'Reilly, for example, dabbled in releasing their ebooks as apps.
About 18 months after the initial launch, Aldiko was relaunched in late 2011 as v2.0. This new version included support for Adobe DE DRM as well as an ebookstore. The developers are working on a v3.0 release at the moment, with even more new features.