As far as I understand it, G+ is still invitation-only ... so the risk of you getting uninvited companions is somewhat limited. Although I think once that barrier is removed, the spammers may start trying to get onboard again.
G+ is obviously positioned as a competitor to Facebook. However whilst Facebook is a self-sustaining platform, G+ is more of a tool for Google to use to encourage uptake and stickiness of users to the overall Google platform (search, docs, mail, youtube).
Although I don't have a personal account on either platform , my understanding is that G+ gives you more granular control over the sharing of your information, and has more focus on productivity features.