If we look over the last 40 years, the rate of big earthquakes has increased. From 1971 to Dec 25, 2004, there were no magnitude > 8.5 earthquakes, whereas since then, there have been 5. However, this is an example of statistics of small numbers. In the 10 years from 1952 to 1965, there were 7 earthquakes above 8.5. Before that time, we may not have had the ability to measure the truly big earthquakes.
Here is a picture of the rate of earthquakes above 8.5:
But, again, this in only showing 15 earthquakes. And not all earthquakes above magnitude 8.5 are created equal. If we look at the total seismic moment of all earthquakes recorded, which is probably proportional to the energy, we see that 1/2 of the moment since 1900 occurred between 1952 and 1965, and about 1/4 - 1/3 occurred in one earthquake: the May, 1960 Chile Earthquake (Mw=9.5)
from: http://www.iris.edu/news/events/japan2011/images/ModifiedPAGERCumMoment-ShallowEQ-Apr2011.png
This shows that the background rate of earthquake slip is relatively similar, except it is dominated by slip from the truly great earthquakes: 1952 Kamchatka, 1960 Chile, 1964 Alaska, 2004 Sumatra, and the recent Tohoku (Japan) earthquake. All of these events were above magnitude 9.0.
As an example, the following video, produced by Dr. Nathan Becker of the Pacific Tsunami Warning Center, shows the relative size of various earthquakes. I think this is one of the best visualizations I have seen.
What this showed us is that these biggest earthquakes are responsible for most of the energy release. And there are few enough of these instrumentally recorded that we can not make any conclusion regarding the long term rate of earthquakes.
Here is a link to Dr Nathan Becker's discussion of earthquake magnitudes:
https://sites.google.com/site/nathancbecker/presentations/earthquake-size-and-frequency-part-3
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