Link me, I'll link you back. [On a blog]
This is a conditional.
If you link (to) me, (then) I'll link to you.
The "then" can't be implied if it's a correct sentence. The conditional and verbs taking the dative is fine, but it's a comma splice. There needs to be a conjunction or a semicolon.
Link me; I'll link you back?
I chose correct, but only because it's clearly intended to be informal. I wouldn't say it was correct in a more formal format.
Link me, and I'll link you back. It's two full sentences simply smushed together in the original form