A person who sucks the social life out of the people they are around. Generally a needy, clingy person. This person may also assimilate parts of the group, after only one or two encounters.
She's so draining; I'm not going out if I have to hang out with that social vampire.
A person that goes around and 'attaches' themself to a person and piles on the affection and attention until they feel satisfied that the person really likes them in return. Once that happens, the person inexplicably moves on to the next 'target' nearly instantly and begins the cycle all over, leaving the last 'victim' confused and maybe a little hurt. These people feel like they're not popular enough and/or need more attention and this social vampirism is their defensive mechanism.
She is such a social vampire; she like just stopped talking to this guy almost overnight and started talking to someone else.