I just checked web sites from the Census Bureau, the Election Project, and the Wikipedia page on the 2016 election. I boiled it all down to this, in very round numbers:
325 million people live in the USA right now
220 million are eligible to vote (most of the rest are under 18)
136 million actually voted
So here's how the 220 million eligible voters actually behaved:
66 million people voted for Clinton
63 million people voted for Trump
7 million people voted for another candidate
84 million people did not vote.
So 'did not vote' won the election by a landslide!
The 66 million people who voted for Clinton are in a distinct minority; 66 out of 220 is only 30%! The same is true for Trump, only about 29% of voters voted for him. 91 million people (41%) didn't vote for either of them. To put it more bluntly, only 29% of eligible voters voted for Donald Trump, compared to 71% who did not.
I'm going to guess that almost all of the 7 million people who voted for another candidate did not expect their candidate to win. We cannot know how many of those 7 million can be considered a 'protest' vote. What I can say is that voting for another candidate or not voting at all had the same effect on the outcome: No effect.
We don't know how many of the 84 million outright refused to vote as a protest, either. Perhaps some of those actually tried to vote, but were blocked from voting by various hurdles. I'll bet that more people decided not to vote out of despair with all the problems in government because they didn't expect that their vote would change any of that. Or they were just so disgusted with the two major candidates and all the negative campaigning that they just saw no reason to vote for either of them (and certainly no reason to vote for any of the other third party candidates).
There may be a significant number of people who correctly reasoned that their one vote out of 220 million eligible voters is so incredibly insignificant, that it doesn't really matter. After all, the odds of being struck by lightning in the US are one in a million, so the odds of your vote counting is 220 times less likely! Kinda makes the slogan 'every vote counts' seem a bit misleading!
You see, I think it's seriously wrong to blame the 91 million people who didn't vote for one of the two major candidates as being lazy or apathetic. It think many of them are just as angry or despondent as many Hillary supporters are right now, now that she lost.
My point is, we are not running the democracy we might think we are running, here. And I think that deserves more serious attention than it's getting. Our system does not really represent the majority of the people anymore.
We can impose term limits (of course, the president is already term limited), or try to get big money out of politics, but none of that will change the fact that one out of 220 million is extremely poor odds at making any difference. And it will only get worse, as our population keeps growing, with more and more people eligible to vote every year.
My recommendation is to find ways to focus much more on the local level, through voting or participating in networks or groups or other action, outside of government altogether. The USA has become Too Big To Manage and Too Complex To Comprehend (for most of us).