In RIA, Flash is the dominant player while Silverlight and JavaFX are new. However, Silverlight is in a better position than JavaFX.
.Net is a huge platform like Java. However, Microsoft trimmed .Net into a 5MB version to become a Silverlight runtime. On the other hand, Sun only added scripting syntax to Java and reuse the whole Java runtime as well as the branding. In fact, Java applet and Java web start has become the burden of the Java family. Users perceive Java is slow, insecure, and has compatibility issues whether or not any of them are true. So, it is no longer a technology issue. It is a marketing issue.
In fact, when it comes to marketing, Microsoft does it far much better than everyone one else in the technology industry.