Answer: roughly less than half as of September 18, 2008.
Methodology:
The pivot shows there are more users not having a picture than users with pictures in every locale (0 means no pic, 1 means has a pic)
Samples: 6508 users from Facebook database table USER
Code: $FQL2="SELECT first_name, last_name, pic_square, locale from user WHERE uid=$xxx"; $resultset2 = $facebook->api_client->fql_query($FQL2);
Userid are created by a hash function in Facebook.
The uid $xxx in the sample set ranges from 1050170000 to 1450180000