Actually in this particular case, you have to use the combined population of Pandas in both factions as they are available to both. That puts EU pandas at 4.5% and US at 4.4% ,
Which makes them more popular than:
Worgen
Gnomes
Goblins
Dwarves
Certainly not the least popular race.Overall, only 3 alliance races are more popular than Pandas, while 5 horde races are more popular than them ( you have to use the combined number for this)