Never heard of it in Am.Eng. either. "Legit" is a slangy shortened form of "legitimate".
I can imagine "legit(imate)" as a replacement for "truly", but only in the slang of some social group that I haven't met yet.
I read it on Facebook as part of an image caption. After some Google search it's clear that it's chat speak. Most of the hits are Facebook or Tumblr or blog posts and comments. Even if it's not standard English I'm curious how words in "legit so excited" could work together. Maybe someone that is familiar with the meaning will come by and shed a light on it.
"Legit" has been used as slang shortening of the adjective "legitimate" for quite some time (for example: MC Hammer's "Too Legit to Quit" from 1991). This shortening of the adverb "legitimately" to "legit" is new to me as well, but I have no trouble understanding it.