Your initial "choice" of posting on the forum was determined by your pre-existing brain-state that was curious about the forum.
That and sensory inputs about the situation (the individual's computer, the words on its screen, etc.) In other words, some of what led up to the decision to post was internal to the individual making the choice (his/her interest in posting, interpretation of what was read etc.) and some of it was external (what the other person had written, the fact that the computer was working, etc.)
It's as much innate for you at that moment as your need for sleep. It doesn't prove a thing.
The 'nate' in 'innate' comes from 'natal', doesn't it? That word means 'birth'. I don't want to say that all of our choices were already determined for us before our births.
But yeah, I think that I agree with you in saying that our decision processes are causal in nature, so long as we recognize that some of those causes influence us from outside, and as long as we don't try to spin causality into fatalism.
I don't think that most believers in free-will (I'm one, I guess) will have much problem agreeing that if we duplicated a situation precisely, including not only the exact external situation that the actor finds him/herself in, but also the actor's own desires, fears, memories, knowledge, assessments and so on, that the actor would probably make the same choice again. Agreeing to that doesn't seem to violate most people's intuition of what 'free-will' means.
So the free-will intuition needn't imply some kind of absolute immunity from causality.
What does violate our idea of free-will is the much more fatalistic extension of the idea, the stronger assertion that everything we do was already predetermined even before we were born. The problem there seems to be that the importance of the actor's internal cognitive decision process is now being dismissed and the actor re-imagined as if he/she was merely a puppet.
In other words, what's creating problem for the free-will intuition isn't causality so much as it's fatalism.
So I'm kind of inclined to speculate that the difficulty isn't so much whether we are causal beings or not (I tend to identify our minds with our brain-processes and am probably more of a reductionist than many here), but rather, how far the causal chains that precede our actions extend and how completely they predetermine our choices and actions.
As I've already argued, for short temporal distances, I think that causal determinism is quite strong. If we perfectly reproduce a situation, down to the actor's own brain-states, he or she will probably behave the same way. But for longer temporal distances, I speculate that the one-to-one correlation between preexisting states and subsequent events becomes more and more fuzzy and stochastic, fading eventually into randomness. So even if we described the physical state of the universe at some point far in the distant past with all of the exactness physically possible, we probably still wouldn't be able to predict what particular humans are doing right here, right now. If we reproduced the universe's state back at the big-bang or whatever, even with all of the precision that's physically possible, in my opinion it's highly unlikely that the universe would evolve the same way a second time that it did the first.
If the more distant past hasn't already predetermined everything, that means that causal beings like ourselves might not be puppets at all. We might have to kind of play it by ear, ad-libbing (however causal that process is) in response to events as they occur around us, in real time. And the need for us to make those kind of choices and decisions, and our ability make them, seems to be precisely what many of us think of as free-will.
So I'm a compatibilist, I guess.