Some sources have claimed that numbers above 900 were used when some state programs were converted to federal control, but current SSA documents claim no numbers above 799 have ever been used

As of Feb 10, 1999 the most recent area numbers to have been assigned include 650-658, 667-675, and 680. This list is from the SSA's web site, which shows the highest group number assigned for each area

The group number is not related to geography but rather to the order in which SSNs are issued for a particular area. Before 1965, only half the group numbers were used: odd numbers were used below 10 and even numbers were used above 9. In 1965 the system was changed so assignments continued with the low even numbers and the high odd numbers. So, group numbers for each area number are assigned in the following order

Odd numbers, 01 to 09
Even numbers, 10 to 98
Even numbers, 02 to 08
Odd numbers, 11 to 99
Group codes of "00" aren't assigned

In each region, all possible area numbers are assigned with each group number before using the next group number. This means the group numbers can be used to find a chronological ordering of SSNs within a region. When new group numbers are assigned to a state, the old numbers are usually used up first

SSA publishes a list every month of the highest group assigned for each SSN Area. For example, if the highest group assigned for area 999 is 72, then we know that the number 999-04-1234 is an invalid number because even Groups under 9 have not yet been assigned

Serial numbers are assigned in chronological order within each area and group number as the applications are processed. Serial number "0000" is never used. Before 1965, when number assignment was transferred from field offices to the central office, serial numbers may have been assigned in a strange order. (Some sources claim that 2000 and 7000 series numbers were assigned out of order. That no longer seems to be the case.) Currently, the serial numbers are assigned in strictly increasing order with each area and group combination

Any SSN conforming to one of the following criteria is an invalid number:  Any field all zeroes (no field of zeroes is ever assigned)
First three digits above 740

The Social Security Administration publishes a list every month of the highest group that is assigned for each SSN Area. For example, if the highest group assigned for area 999 is 72, then we would know that the number 999-04-1234 is an invalid number, because the "even" Groups under 9 have not yet been assigned.  See the latest Social Security Number Monthly Issuance Table for the latest SSN area ranges issued to date.

Serial numbers are assigned in chronological order within each area and group number as the applications are processed. Serial number "0000" is never used. Before 1965, when number assignment was transferred from field offices to the central office, serial numbers may have been assigned in a strange order. (Some sources claim that 2000 and 7000 series numbers were assigned out of order. That no longer seems to be the case.) Currently, the serial numbers are assigned in strictly increasing order with each area and group combination. (Social Security Numbers are currently assigned by computer in Social Security Administration headquarters in Washington, DC. There are relatively rare cases in which the computer system can be over-ridden by manual assignment --- such as a recipient refusing a number containing the sequence 666.) SOCIAL SECURITY: YOUR NUMBER