There are infinitely many ways to do what you want. For example, you can rank the channels by giving each one a score, and then rank them from highest to lowest score. There are infinitely many ways to so that, to:
* $s=\\#\text{active people}$
* $s=\\#\text{active people} \cdot \\#\text{messages}$
* $s=1000\cdot \\#\text{active people} + \\#\text{messages}$
* $s=\\#\text{active people} + \sin(\\#\text{messages})$
and so on and so on.
Now sure, some of the functions look stupid (why would you use a $\sin$ function?), but they give the ranking you want, and with no further detail, that's all you can say.