Are any airlines actually flying the Comac ARJ21?
You are correct in that the FlightRadar24 site doesn't show any flights for the ARJ by aircraft. But going to the routes tab and searching through the routes there are ARJ assigned to some routes. So it appears that Chengdu Airlines is operating the aircraft at least on a limited basis.  

I would also say that while FlightRadar24 data is generally very good, it isn't perfect. They do rely on data from external sources and considering the Chinese government's information and internet policies, it's entirely possible that the data availability is not the most reliable.