It's disappointing to hear about low numbers on the AMS route.
Heard from another source that the reason for the big drop in passengers is that easyjet have now realigned the prices of some flights so that they are more in line with their services from other London airports.
It seems that they have no problem filling seats at Southend but only if they keep prices artificially low. The thinking is that these passengers would have booked a higher price ticket at another London airport so easyjet were being hit twice by not getting the higher fare and by subsidising the ticket price from Southend as part of the route launch.
With similar prices Southend appears to have lost some of its appeal despite the benefits of a small terminal. Not sure how this will play out in the long term and also I am only going by what I have been told.