Depends on what your definition of "big" is.
Quoted from heckheck:
It's a bigger hobby than you might guess. It would be interesting to know how many machines are still in circulation. I bet it's well in excess of 100K units.
Some population stats for comparison:
Top 10 Most Populous Countries
Rank. Country Population Rank. Country. Population NaN
1. China 1,349,585,838 6. Pakistan 193,238,868
2. India 1,220,800,359 7. Nigeria 174,507,539
3. United States 316,668,567 8. Bangladesh 163,654,860
4. Indonesia 251,160,124 9. Russia 142,500,482
5. Brazil 201,009,622 10. Japan 127,253,075
Ford sold 83,000 Mustangs last year, that's .03% (I think? My math is awful) of the US pop buying a Mustang last year.
Ford made their 1,000,000th Mustang in April of 2013. If say 50% of those were still running/owned, that would be .16% of the US population if each person owned one.
I imagine that the number of Mustang owners is higher than Pinball owners. Although .1% of the entire US population is pretty big as a size, it's still pretty rare that you're going to come across another one in the wild.