Dual_EC was the default PRNG for the RSA BSAFE library. The only actual numbers I've seen regarding how popular the library was this researcher's findings[1] in which he did a scan of 21.8 million IP addresses and managed to find 720 servers using it (to be fair, that's a lower bound as there were two implementations of BSAFE and only one was detectable remotely). It was generally easier use a PRNG provided by the OS or use an open source library for free. As a testament to how unpopular Dual_EC was, there was a bug in OpenSSL for years that prevented it from working at all when Dual_EC was enabled, and it wasn't discovered until after Snowden.