(a) Google isn't like "nearly every consumer startup". It is arguably the most powerful Internet company in the world. Thus its actions must be viewed differently.
(b) In this case Google is using its dominance in the search space (70-90% in many markets) to push G+ on customers who may not normally have signed up for it.
While it's true Google enjoys a near monopoly (maybe an actual monopoly, I'm not intending to split that hair) but it's very important to make the distinction between a natural monopoly and an illegal monopoly.
(b) In this case Google is using its dominance in the search space (70-90% in many markets) to push G+ on customers who may not normally have signed up for it.