... as in the internet address, not the concept of an IT company...
Some web site with an impossible name posts the list, the first few of which are here:
SYMBOLICS.COM in March 1985
BBN.COM
THINK.COM
MCC.COM
DEC.COM
NORTHROP.COM
XEROX.COM
SRI.COM
HP.COM
BELLCORE.COM
IBM.COM
SUN.COM
INTEL.COM
TI.COM
ATT.COM
GMR.COM
TEK.COM
FMC.COM
UB.COM
BELL-ATL.COM in August 1986