Most rape victims don't tweet about how it's "amazing" to hang out with "cool" and "smart" people after being raped, or throw a party for their rapist.
http://rixstep.com/1/20101001,01.shtml
Nor do they usually invite their rapist to stay at their house 4 days after the rape.
http://nymag.com/daily/intel/2010/12/crayfish_parties_and_br...
It's also a rather implausible coincidence that after these events, the rape charge only surfaced after a) Assange failed to call either Anna Ardin or Miss W. back and b) Ardin and W each discovered he was sleeping with the other one.
I think the story itself justifies a very low probability of being true - in my estimation, probably considerably less than 1%.