I haven't noticed a pop drop on the Bastion. There's almost always at least 2 fleets and Nar Shaddaa and Coruscant have both been really full this last week with the event going on.
On non peak hours we're lucky to have 150 people on fleet and during peak we usually have 2 instances but the second is never full anymore. For like the first 3 weeks after launch it had 3 instances all the time. The pop is dropping again, even though a lot are still on, and will continue until new content is added (maybe).