It's best to have a pervasive presence, but if you have to choose a platform go with Facebook. Facebook has supported businesses longer through Pages and allows you to run campaigns targeted at a specific demographic easily right on the site. Google+ is a bit immature. It's also rather unpopular.
My experience of Google+ is that the membership is mainly comprised of people who also use Facebook or Twitter, and took a look when the new boy in social media started up. They have not changed their primary usage patterns, and Google+ is not becoming a thriving community. It's something of a vicious circle: a social network needs lots of active members to succeed, new members join and discover there's not much going on so leave (or just dip in every now and then), the social network never reaches the required critical mass or membership momentum.
Facebook has that critical mass in terms of active members. It's where my friends are, and despite some also using Twitter, LinkedIn and Google+ the only place that they are ALL together as a group is on Facebook. Consider that this is the same for the majority of social circles and that's a pretty hard nut to crack for any new service trying to break the stranglehold.
As I've stated elsewhere, Internet giant killing is not impossible (AltaVista, Netscape, MySpace etc etc) but neither is it easy. For now, Facebook is the primary player in social media but the clever money spreads the bet and has that pervasive presence that dmanw100 speaks of.