Thanks for your comment Robin and sorry for just replying now,
I think between different Google products its going to become hard for users to avoid signing up for Google+. While I can't see it ever becoming compulsory to sign in to use products like search, maps or YouTube, I'm sure Google will find ways to get more people signed into accounts. Unlike Facebook, who rely quite heavily on users inputted profile information, Google already have a wealth of information on users without them having to specify t in their Google+ profiles, so even if a person joins G+ and just enters the most basic information (name, dob etc) Google can easily associate huge amounts of other data with their profile i.e.
- Location - from their IP location
- Gender - from their name
- Interests, profession, social grouping - from their search history, youtube views
- Connections - from their gmail and hangouts
This is why I don't think Google are too worried about people not using G+ for social networking, the key thing is getting them to have an account and they can then link huge amounts of data to that user account with or without the users input.