The reason that Google and Facebook were not included was essentially that there was not enough data for the month for these two sites to be included.
Google had been included in the April benchmarks, but because of technical changes to their application, there was not enough data gathered for a proper comparison, and they could not be included in May's benchmarks. From time to time a firm's data can't be included because of technical peculiarities such as this, and/or the ability to generate a full month’s worth of data. So there will inevitably be firms missing occasionally but the goal is to be as complete as possible, so it's possible we may see them in next month's benchmarks.
