Facebook .. because that's where everybody is. Especially good for marketing to the high school seniors.
Twitter .. sucks for seniors and general portraiture .. we limit twitter efforts to our boudoir services.
Google+ .. sucks for everything right now but we are building it anyway because it rocks and we're hoping that it becomes a major player with the general consumer. We have multiple G+ pages now streamlined different client types. ... As of now we have got a few clients through G+ and they have been business executives (not for headshot sessions but that's the income class they fell into)
LinkedIn .. brings in some quick n easy business headshots but that's mostly all we see from it.
MySpace ... MyWhat?
ModelMayhem ... very few model portfolio and boudoir requests. Primarily filled with wannabe models. We use it when we need to scout a model for a commercial shoot, or our own stock shoot .. in which cases we are being compensated by a third party so the all popular TFCD method that the wannabes are use to works out ok for us.
Flickr .. Awesome if you're client demographic is other photographers .. same goes for 500px