So I did a few searches, and these are my results:
- fact vs. fiction: fact occurs much more often, but has taken a decidedly sharp down turn since the early 1970s. Also, fiction is on the rise.
- happy vs. sad: happy is on a serious decline. It bottomed out in the mid-late 1980s (a little cold war scare anyone?), but does seem to be on the rise again. Sad rose to a height in the late 1860s, but has been on the decline, except for a small rise in the late 1920s, ever since.
- ain't vs. isn't: these two words have an interesting relationship. there was a time just before 1900 that ain't was more prevalent. Then rightfully so isn't was used more often. Then just after 1940 they both enjoyed a wild spike in usage and both shared a huge dip in usage in the early 1960s. Also ain't remains in print, isn't has spiked beyond its 1940 era boom - thankfully.
- yes vs. no: is a most interesting graph. It seems that yes has never had much ground, and no was very famous. However no has been steadly decreasing since a peak around 1840.
- pencil vs. pen: it looks that the pen has had the upper hand for two hundred years, but both are in a steady decline.
- disco vs. funk: beginning in 1970 funk began in the lead and then there was a crutial tipping point in 1976 and disco soared while funk stagnated. However, funk enjoyed a resurgance in the 1990s, but by the time 1999 rolled around it was on the down beat again.
The homepage is http://books.google.com/ngrams/ - go explore.