AN ACR FO (PF) LANDED AN MLG PAST THE STANDARD TOUCHDOWN AREA ON A SHORT RWY. AFTER EXCHANGE OF CTLS; THE CAPT RECOGNIZED THAT THE BRAKE EFFECTIVENESS WAS INADEQUATE AND THAT HE WOULD BE UNABLE TO STOP THE ACFT BEFORE THE END OF THE RWY. THE ACFT WAS TURNED TO THE R TO AVOID THE LOC ANTENNA ARRAY PAST THE OVERRUN. LNDG OVERSHOT ON WET RWY. RWY EXCURSION.

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I WAS ACTING AS PIC OF A FLT FROM ATLANTA TO SAVANNAH WITH THE FO ACTING AS THE PF. THE FLT ENCOUNTERED INST METEOROLOGICAL CONDITIONS AND OCCASIONAL LIGHT TURB FOR APPROX 20 MINS UNTIL DSNDING FOR THE APCH INTO SAVANNAH. (RWY 36 IS 7000 FT LONG.) APCHING SAVANNAH WE RECEIVED THE ATIS; BRIEFED FOR AN APCH TO RWY 36 AND READ THE DSCNT AND APCH CHKLISTS. THE FLT ENCOUNTERED VFR CONDITIONS ONCE THROUGH APPROX 11000 FT AND WITHIN ABOUT 10 MI OF THE ARPT. WX WAS VFR WITH APPROX 3 KTS TAILWIND COMPONENT. WE WERE VECTORED BY APCH FOR A L DOWNWIND TO RWY 36 WHERE WE CALLED THE RWY IN SIGHT AND PROCEEDED WITH A VISUAL BACKED UP BY THE ILS. THE BEFORE LNDG CHKLIST WAS READ AND ALL PRELNDG ITEMS ACCOMPLISHED AND CHKED. THE APCH WAS STABILIZED AND NORMAL INTO THE FLARE. AT THE TIME OF THE FLARE ALL CONDITIONS APPEARED NORMAL; HOWEVER; THE PLANE FLOATED WELL PAST THE NORMAL COMPANY STANDARD TOUCHDOWN AREA OF 1500 FT FROM THE THRESHOLD. THE EVENTUAL TOUCHDOWN WAS AROUND 2700 FT FROM THE THRESHOLD. THE GND SPOILERS DEPLOYED NORMALLY AND REVERSERS BROUGHT OUT APPROX 65 PERCENT L N1 AND 75 PERCENT R N1. THE ACFT SEEMED TO DECELERATE NORMALLY FROM THIS POINT; HOWEVER; THE END OF THE RWY WAS NOT YET VISIBLE DUE TO THE GRADE OF THE XWIND RWY. THE ACFT DRIFTED L OF CTRLINE AND I INSTRUCTED THE FO TO CORRECT IT. AT ABOUT THIS POINT THE END OF THE RWY BECAME VISIBLE AS WE CROSSED THE XWIND RWY AND THERE APPEARED TO BE SUFFICIENT RWY REMAINING TO STOP THE ACFT. THE RATE OF DECELERATION; HOWEVER; APPEARED TO DECREASE. ACFT CTL WAS THEN TRANSFERRED TO MYSELF AND I BEGAN HVY BRAKE PRESSURE; BUT WITHOUT THE DESIRED EFFECT. AT THE TIME I TOOK CTL THERE SEEMED TO BE SUFFICIENT RWY REMAINING TO STOP; BUT BRAKE EFFECTIVENESS WAS INADEQUATE. THE ACFT PROCEEDED INTO THE OVERRUN AREA AND THEN OFF INTO THE DIRT FILL BEYOND FOR ABOUT 100 FT. WHEN IT WAS APPARENT THAT THE ACFT MIGHT NOT STOP ON THE PAVED SURFACE I TURNED IT R TO AVOID CONTACTING THE LOC ANTENNA ARRAY JUST BEYOND. AFTER STOPPING THERE WAS NO APPARENT DANGER SO PAX WERE DEPLANED OUT THE FORWARD AIRSTAIRS IN AN ORDERLY MANNER. NO DAMAGE OCCURRED TO THE ACFT. THE LIKELY CAUSE WAS FO LNDG LONG ON A RELATIVELY SHORT RWY AND CAPT NOT TAKING CORRECTIVE ACTION SOON ENOUGH. EARLIER RAIN MAY HAVE AFFECTED BRAKING ACTION SINCE BRAKING WAS NOT AS EFFECTIVE AS EXPECTED. THE FO SAID AN ANTI-SKID LIGHT WAS ON; BUT NO FAULT WAS SUBSEQUENTLY FOUND WITH THE SYS. I RECOMMEND HARD STANDARDS FOR LNDG ACCURACY AND IMMEDIATE RESPONSE (BY GOING AROUND) FOR NON-COMPLIANCE.