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The milestone metric and the accelerating signup rate are especially telling given StumbleUpon's roller coaster history -- once an eBay-owned company, StumbleUpon returned to startup status in April 2009.
Since the dissolution of the eBay-StumbleUpon marriage, the company has worked to rebuild its stature with product enhancements, including a recent upgrade to its video product. These upgrades seem to be contributing to a renewed interest in the service.
At this rate, the serendipitous web discovery engine appears to be successfully transitioning from relic service to trendy startup -- an impressive feat considering it launched in 2001.