From BTS's T-100 segment database, these are the top 10 airlines at BUF for Jan-Aug 2018.
CARRIER_NAME PASSENGERS Percentage
Southwest Airlines Co. 1069949 31.37%
JetBlue Airways 545362 15.99%
Delta Air Lines Inc. 377479 11.07%
Republic Airline 235222 6.90%
Endeavor Air Inc. 232027 6.80%
American Airlines Inc. 183804 5.39%
Frontier Airlines Inc. 169029 4.96%
SkyWest Airlines Inc. 102827 3.02%
United Air Lines Inc. 98112 2.88%
GoJet Airlines LLC d/b/a United Express 77479 2.27%
Quick notes comparing to 2017
Republic's usage is way up, American is up a bit, Frontier has captured a good chunk of the market, United mainline is up, GoJet usage is way up, Envoy has fallen quite a bit.
All airline stats seem to make sense. Southwest and Jetblue leading the way ..... The Legacy carrier never seem to expand their routes out of BNIA. (Yes, I know AA has added seasonal Miami for the spring and adding DFW. )
United and Delta, I honestly cant think of a seasonal or added route in recent memory.
Frontier will always be a wildcard. Due to them changing routes every few months. (It seems like)