Well obviously the spongebob guy watches Korra, and there's no way a pony fan dislike Korra.
Extremely likely is all three are fans of MLP, and perhaps the pre-film Spongebob.

In other words, a room full of normal people with varied interests who happen to sport a specific one more prominently at the moment.

well this image is quite the melting pot of fandoms.
we've got spongebob taking away Lin's bending, while in the background some brony's just handing out.