I don't understand what is "membership" in sites like Google/Twitter/Facebook. I thought that it could be when you have an account in there vs you don't; but how then can you be a "non-member" at Facebook, for instance.
Many people regularly use Google without signing in.
Twitter is admittedly more membership based, but they still have millions of people viewing tweets (especially embeds) without an account.
Facebook is indeed primarily a logged-in experience, but if you look at the numbers you'll see that uniques are less than members. So it's essentially a measure of how many members actually visited that month.
Without knowing the methodology used in the OP, I occasionally visit Facebook (eg, most of the cafes in my area are easier to find on FB than their own site, so I can check opening hours; some businesses post articles or information publicly on FB that's relevant to me) but I don't have an account anymore.
Can someone please clarify this for me?