Less than 2% according to Facebook's last 10-Q: "We also seek to identify "false" accounts, which we divide into two categories: (1) user-misclassified account

Less than 2% according to Facebook's last 10-Q: "We also seek to identify "false" accounts, which we divide into two categories: (1) user-misclassified accounts, where users have created personal profiles for a business, organization, or non-human entity such as a pet (such entities are permitted on Facebook using a Page rather than a personal profile under our terms of service); and (2) undesirable accounts, which represent user profiles that we determine are intended to be used for purposes that violate our terms of service, such as spamming. In 2014, for example, we estimate user-misclassified and undesirable accounts may have represented less than 2% of our worldwide MAUs."