My point is: We can only count passively. That means: After a month has gone we can't accurately count the number of active threads easily, since some threads tend to stay active beyond a given month. And even I won't go in every thread and look if it has been active in a given month.
We can count killed threads which give us an impression on how many threads a last post was made. This is only possible post mortem and it has been done some time. I will give a summary of 2012 in March when the last results are in.
Since this discussion was already begun some time ago I added at that statistics the birth-month of threads. It turned out so far that most threads die the month they are created and a small percentage makes it over to the next month (mostly threads created late in a month). Very few live for several month, though we hacve a few which made it to years!
Now I went and look up how many threads were born in January 2012: The number was a whooping 650! Thread 144268
to Thread 144918
In January 2013 we have a meager 535 Thread 150569
to Thread 151104
However as I pointed out some time ago some threadid's don't exits (probably when people create a new thread and decide not to post it after all.)