Given how plastic the brain is I suspect that the limit on social relationships is more logistical than physiological.
ie actually meeting, being social for long enough to develop a relationship and then maintaining that relationship with someone takes a certain amount of time. Above a certain number of relationships the individual simply doesn't have the time or incentive to acquire more.
If there was a real incentive for someone to have more than 150 relationships I would expect that either a), their brain would rewire itself somewhat to handle the increased number or b) they'd make abstractions that would help them to maintain friends more efficiently.
Facebook might be a way to circumvent Dunbar's number. Knowledge is stored on computers, rather than in your brain, and you can decide to retrieve it if needed.
