Facebook achieved this goal in April of 2010, but StumbleUpon was already well on its way.
At that time, StumbleUpon already gave twice as many referrals as Twitter.
StumbleUpon's user experience is fanciful and fun, but its traffic power for publishers is quite serious.
While the other social networks make the headlines, StumbleUpon has been a quiet success story.
In light of today's news, it won't be so quiet for long.
Source: StatCounter Global Stats - Social Media Market Share