I will win
Not Immediately
But Definitely
Some local Multi Level Marketing pimps are posting this glurge on Facebook to show what hardcore achievers they are.
Seems like a real clunky paragraph (if looked at as a paragraph)
Is this good English or not?
Needs to be tightened up: “I’m losing right now” says the same thing.
Needs punctuation.
OK glass half empty…lol. 
Well, if I want to be a dick about it, I’ll point out that it entirely lacks punctuation and, given the separation into three lines, looks like a very simple sentence, followed by two sentence fragments. Also, they need to brush up on the rules of capitalization, which really aren’t that difficult–we’re not talking about the apostrophe here, Batman.
If I go the other way and give it the most generous reading, it is the following: “I will win, not immediately, but definitely.” That’s a bit clunky, IMO, but appears to be a perfectly cromulent English sentence, violating no rule of grammar that I can identify.
Sentence fragments. Good idea. Will be used again later.
I blame William Shatner.
And again. And again.
Yes, provided sentence fragments are acceptable – and headline capitalization – the quote in the OP is fine grammatically, but not (in my opinion) the best stylistically.
I’d revise to something like:
I will win.
Perhaps not now.
But in the end.
(Shorter words usually have more impact.)
As it’s presented, both here and in the link, I don’t fault the punctuation–it’s almost “bullet point” style.
But I do wonder whey Immediately and Definitely are capitalized, but “will win” is not. I call the apparently purposeless capitalization of certain words “Pooh case,” because Winnie the Pooh talked that way.