It was used in the pre-anime material (Dengeki write-ups, possibly also the drama CDs, I don't recall), as well as the game at least once. Yoshiko's also called her Ricchan, and the Dengeki stuff has Riko calling her Yocchan. Yoshiko supposedly drags Riko into Numazu on weekends to go clothes shopping and do karaoke and stuff, according to the same material.
Also, Rikyako and Aikyan play this shit up like mad, so...it sticks.
