Two political scientists, Matthew Hindman and Bernard Tamas, have this analysis in Monkey Cage in the Washington Post. They show that there are more minor party and independent candidates for federal office now than there were fifty years ago. They are correct. However, they could have noted the drop-off in the number of such candidates on the November ballot starting in 2012, due to the top-two systems in California and Washington, and they did not. In 2014 over half of U.S. House races had no minor party or independent candidates on the November ballot. Thanks to Rick Hasen for the link.

From the WP:

The evidence below suggests that third parties are rebounding. As the graph shows, 50 years ago, third-party candidates made it onto the ballot in fewer than 1 in 10 races for the U.S. House of Representatives. That number has risen fairly steadily since. Over the past 20 years, well over half of all U.S. House districts have had a third-party candidate on the ballot.