rish3 - 10:16 pm on May 8, 2013 (gmt 0)
The Yelp example seemed interesting to me, so pulled some Comscore numbers from the "Top 50 Properties" reports they release. The data is fairly simple...Comscore provides the "total # of unique visitors" across all properties, then the "# of unique visitors" for each one individually.
Since the total varies month to month, I converted the numbers into "percentage of ALL visitors, to all sites...that visited this property".
Just three data points (Dec 2012, Feb 2013, and and Mar 2013), so this isn't definitive, but it's certainly interesting. Yelp is gaining "share" at a pretty good clip, while G, Y, etc, are roughly flat.
The plotted chart (jpg) [i.imgur.com]
The data (jpg) [i.imgur.com]