The question is "is it here to stay".
Take Ruby on Rails. Was the craze not even half a decade ago. Everyone was on Rails. Too bad they led to the chasm and nobody bothered to build a bridge over it.
So learning a language because some startups are crazy about it isn't worth it. But what is? How can you tell whether a language "gets big" or is a tempest in a teapot?
Easy. It ain't the language, it's the people using it. It's the movers and shakers of an industry that decide what will grow and what will perish. If Bruce Schneier started writing his code snippets in Splfurt (I sincerely hope there is no such language, I just made that word up) and if people from Metasploit pick it up and code their stuff in Splfurt, Splfurt is the new big thing in IT security and every framework, scanner, tool and whatnot will have to have Splfurt plugin support and new exploit PoCs will come written in Splfurt.
It's not the language. It's the people using it.