A person who absorbs the personality and interests of the people around them like tofu absorbs the flavor of the food it is cooked with. They usually do this because they lack a personality of their own, and they will often forsake their previous persona for a new one at any given moment.
That guy is Social Tofu, sure he'll talk to you about the injustice of animal abuse but tomorrow he will defend bear baiting to the death.
Someone who always changes their personality to match the crowd they are hanging out with -- such as tofu taking on the flavors of the food around it.
Person 1: Jeez, does she have her own personality? She seems to have like 15 depending on the time of day..
Person 2: No man, she's social tofu, just mimicking whoever she's around at the moment.
A person that neither adds or takes away from a social situation only bringing physical presence to the event.
Andrea wants a big party so she invited the Jerry and his friends in the debate club as social tofu.