Engagement does matter and offers an important metric for measuring the health of a community... specifically the level of enthusiasm community members have for what they are participating in.
I suppose I will kick in the possibility that POB is a fairly new community, and we may just be reaching the stage in the natural development cycle in which "the novelty factor has worn off." I'm hesitant to bring token price into the picture, except to point out Hive overall generally sees a slowing down in activity level when the token price dives... and POB has gone from better than $1.00 to 25c-ish... and that will have an impact.
Good reason to become extra active, though... and build your stake while prices are low!
=^..^=
Yes this looks quite obvious to me too. When the prices go down just like any other professional, the contributors also think about the return on investment they can achieve from the time they spend on the platform. If the prices are low the justification of time becomes difficult and naturally the engagement goes down.