.... let's rephrase your (provocative) statement to "Belfast City to be 3rd biggest (by passenger numbers) on the island of Ireland".
Now you're getting pedantic!
I take your point about extrapolating August's increase as not necessarily being a valid approach, but I didn't have any other figures to go on. However new figures released this morning show a 12.5% increase in November, so something is happening at that airport. It is definitely bucking the trend.
Personally I'm from Derry and so am more interested in LDY but I just noticed BHD was about to supplant Cork and Shannon. If Ryanair pull the plug on Shannon then all is lost, but Cork is not so dependent on a single operator and so the future is not as bleak. Indeed Belfast City is capped by the planning authorities at 4 million passengers per annum so Cork could regain its title so long as the passenger cap is not overturned which is what BHD is currently trying to achieve.