Complex sentence: StumbleUpon said that before the end of May 2008, it would have collected its five-billionth "stumble", more than one billion of which would have taken place in 2008 alone.
Split into simpler sentences: StumbleUpon said that before the end of May 2008, it would have collected its five-billionth "stumble". More than one billion of which would have taken place in 2008 alone.