This is a great question, but it's really impossible to answer for certain at this point in time. Trump's current success is defying a lot of "conventional wisdom" about how primaries go. That said, the odds are good that your reason #2 is the most likely:
"Donald Trump supporters fall under a category of people with whom I have little contact, at least of the type that would be likely to talk politics."
This article from FiveThirtyEight, while it dates from December (well before any actual votes) still seems accurate when it says:
The latest polls of the Republican presidential primary show a party badly divided by education: Donald Trump’s strong showings are entirely attributable to huge leads among voters without a college degree, while voters with a degree are split among several candidates.
It then goes on to draw a parallel to last election cycle:
A similar diploma divide was starkly evident in 2012, when college-educated Republicans almost single-handedly propelled Mitt Romney to the nomination.
Romney’s two chief rivals, Rick Santorum and Newt Gingrich, combined to win 765,329 more primary votes than Romney before they exited the race in April, thanks to their dominance among voters without college degrees. But those non-college-educated GOP voters split fairly evenly between Santorum and Gingrich, allowing Romney to prevail with a plurality of votes.
Part of what Trump has going for him this election cycle appears to be motivating the non-college-educated-but-conservative segment of the population to turn out and vote in higher numbers than usual. All four Republican contests so far have had greater turnout than in 2012, including Nevada where Trump got more votes this year than there were votes cast in 2012. If Trump supporters are people who generally have not been involved in primaries or politics in the past, then you and your politically-aware circles would have very little overlap with them.
Additionally, since that demographic is showing up specifically to vote for Trump, then the division of the more politically active segments between the rest of the field leaves an opening for a plurality candidate who can never reach a majority. One theory says that Trump has a "ceiling" of somewhere around 35% support (+/- 5%). 35% is a lot in a race where the other 65% is split 22/22/8/8/5, but it's nowhere near enough in a race where the other 65% is united. Even if it were 45%/55% after consolidation, Trump wouldn't be winning.
All that can probably be summed up by saying:
- You aren't finding Trump voters in your circles because you're talking to college-educated people who were already interested in politics.
- Trump is winning because the voters in your demographic are splitting their votes among non-Trump candidates.