Among presidential candidates who had candidates for presidential elector in states containing a majority of the electoral college, Evan McMullin easily received a greater share of his votes via write-ins than any other presidential candidate.

McMullin was on the ballot in eleven states: Arkansas, Colorado, Idaho, Iowa, Kentucky, Louisiana, Minnesota, New Mexico, South Carolina, Utah, and Virginia. These states represented only 16% of the electorate. From those eleven states, he received 510,002 votes. In the states that counted his write-ins, he received 221,267. This means 30.3% of all his votes were write-ins. Never before had a presidential candidate of significance received such a high proportion of his votes as write-ins.

Prior to McMullin, the presidential candidates who received the largest share of their votes as write-ins were Eugene McCarthy in 1976, and Ralph Nader in 1996 and 2004. But neither of them received write-ins amounting to more than one-eighth of their totals.

The final McMullin vote will be increased when Rhode Island finishes tallying its presidential write-ins, which may be as early as January 4, 2017.

“Drop-off”, in the study of elections, means the share of voters who cast a ballot but don’t complete the ballot. For U.S. Senate, in November 2016, the California drop-off was 16.20%. In other words, almost one-sixth of the voters who cast a ballot didn’t bother to vote for U.S. Senate, which was the second item on the ballot, just after president.

This is undoubtedly because the November 2016 California ballot forced voters to either vote for one of two Democrats, or not to vote for U.S. Senate at all. This happened because of Proposition 14, the top-two system, which has been in effect in California since 2011.

The drop-out rate for U.S. Senate in California in the past 75 years has been:
2016 16.20%
2012 4.72%
2010 2.92%
2006 4.02%
2004 4.26%
2000 4.66%
1998 3.55%
1994 4.34%
1992 5.05%
1988 4.42%
1986 2.87%
1982 3.21%
1980 5.10%
1976 8.17%
1974 4.12%
1970 2.13%
1968 3.55%
1964 2.64%
1962 4.75%
1958 .95%
1956 3.36%
1954 4.19%
1952 12.81%
1950 4.15%
1946 4.35%
1944 7.33%

Years not mentioned above had no U.S. Senate election.

According to this story, the voters of Idaho chose four presidential electors for the state on November 8, 2016, who included two federal employees. Afterwards this was noticed, and the two electors didn’t show up at the December 19 presidential elector meeting. When electors don’t attend the meeting, they can easily be replaced, and they were replaced in this case. Thanks to Jim Riley for the link.