First, each spike lasts for roughly two days. But the more interesting fact is that, even though Digg brought in 3.5 times as many viewers than StumbleUpon on the first day, both had a near 50% drop on the second day. The percentage of views on second day to views on first day was precisely 50% on StumbleUpon and 48% on Digg.
This seems to imply one thing: StumbleUpon and Digg are fundamentally the same. They would seem to be different, for they have different modes of operation, i.e. random, recommended pages from a button versus lists sorted by time or popularity, and they have different user bases. The stats, however, show a disturbing similarity.
I wonder whether other sites that get hit by StumbleUpon or Digg experience this phenomenon too.