I'd be curious to see if the population of disillusioned independents is growing faster as well. I'd speculate most of them would be categorized as "moderates" which is a species rapidly disappearing, sadly from both political factions. I for one count myself among them, both parties have developed fundamental show stoppers that make it impossible for me to vote for either candidate in presidential elections. I don't at all consider my vote "thrown away". A vote for a 3rd party is a vote against both, it still counts and enough of them should garner attention for more moderates eventually.
Sadly, no. It does send them the message, but it's a message that doesn't matter to them.
Consider: the actual election itself boils down to which party can send out an image that best registers with the centroid of American public sentiment. The existence of other parties on the fringes has the same effect as individuals too fed up to vote; i.e., it takes votes away which otherwise would have been cast for the one of the two major parties closest to that voter's philosophy. (And no, they're not identical. That's just a sound bite, which is impossible in reality, if for no other reason that there are different people in each party).
But worse: given the electoral college system and the winner take all electoral delegations of most states, the majority of states are a lock for one of the big two parties or the other, so it makes no difference if even a very large number of voters decide to sit it out. The only states that matter are the swing states.
And, just to add to that, the electoral representation of the states isn't directly proportional to population, due to the favoritism for small states baked into the system, so even if you live in a swing state, whether you cast a vote for one of the big two, or another party, or just sit it out has a different effect depending on which state you live in.
I'd say to have the most effect on the government, you need to influence a party's candidate selection process. That's the point where money, efforts, etc for a given candidate has a direct effect; moving to another candidate, or just bailing out, has a distinct negative effect for the candidate you'd otherwise back, and sends an unmistakable, unignorable message. Once the two candidates of the two major parties have been named, the majority of Americans might as well just go take a nap until it's all over and they get notified who the undecided voters in the swing states have chosen for them. Which is kind of scary, given that undecided voters are more than likely idiots.