With the microsoft messenger (MM) data, Eric Horvitz and Jure Leskovec at microsoft calculated that each pair of persons using MM is on the average 6.6 degrees separated.
They calculated this from the data of 180 million MM users. This means that roughly each MM user sends messages to on the average a minimum of 19 different persons. That is when all those persons are different. Since there is a huge overlap between the people I know and the people that I know know the real number will be a lot higher.
This conclusion comes from a very simple spreadsheet of which I show the resulting chart here.

number of people connected / in network

.

You see that the vertical axis is in logarithmic scale.
The two red lines cross somewhere between the 6 and 7 line (to approximate the 6.6 degrees) and on the 180 million people. That gives a figure of 18 on the horizontal axis.

I know this is “spielerei”, but the result seems logical.
I wonder if the people of LinkedIn, Facebook, SecondLife etc… will do the same, just to check the number ?