Found on Twitter:
Il n'y a que sur Twitter que vous pouvez voir une vache manger une banane…
I think that's the first time I've seen "il n'y a que" in the wild. "Il y a" (literally "it there has") is the typical way to say "there is." ("Il y a une vache" is likely what a translator would give you for "there is a cow.")
But this is a negation. "Il n'y a pas un vache" means "there is not a cow" – the phrase "ne pas" is split amid the phrase. "Que" rather than "pas" implies something more like "except." So, roughly, "there is not except on Twitter that you can see a cow eating a banana." More naturally, "only on Twitter," which is what this francophone seems to mean.
"Seulement" is "only," and that seems simple enough to ask why one wouldn't just use “seulement sur Twitter.” Looking through Oxford's English-French dictionary, I can't tell whether that would sound equally fine or slightly unnatural. In any case, the tweeter's phrase sounds like something you'd get used to if you spoke the language – there's a satisfying feeling to that "ne pas" construction with that "except" twist, and the relatively brevity with which you can execute that in French.
(Having said all that, it's also possible on YouTube.)