Currently over at the official website, it says there have been 4011 songs released, 1987 of which come from RBN. You don't need to be a math major to know that's just under half. And at the rate they are going, RBN is likely to surpass official DLC in terms of quantity by the end of the year.
189 of these songs are 2x bass pedal versions, so it might be another few months before the total number of different songs on RBN exceeds the number of HMX-charted songs.
Still, that's a pretty amazing milestone for the platform when it happens.
Expert Pro Keys: 50/63 GS, most recent The Killing Moon
Expert Pro Drums: 53/83 GS, most recent Free Bird / Oh My God / Oye Mi Amor
Expert Pro Bass: 6/83 GS, most recent Everybody Wants to Rule the World