Google Plus is central to Google’s future — a lens that allows the company to peer more broadly into people’s digital life, and to gather an ever-richer trove of the personal information that advertisers covet. Some analysts even say that Google understands more about people’s social activity than Facebook does.  http://www.nytimes.com/2014/02/15/technology/the-plus-in-google-plus-its-mostly-for-google.html?hp&_r=0

The reason is that once you sign up for Plus, it becomes your account for all Google products, from Gmail to YouTube to maps, so Google sees who you are and what you do across its services, even if you never once return to the social network itself.

Before Google released Plus, the company might not have known that you were the same person when you searched, watched videos and used maps. With a single Plus account, the company can build a database of your affinities.

Google says Plus has 540 million monthly active users, but almost half do not visit the social network.

“Google Plus gives you the opportunity to be yourself, and gives Google that common understanding of who you are,” said Bradley Horowitz, vice president of product management for Google Plus.

Plus is now so important to Google that the company requires people to sign up to use some Google services, like commenting on YouTube. The push is being done so forcefully that it has alienated some users and raised privacy and antitrust concerns, including at the Federal Trade Commission. Larry Page, Google’s chief executive, tied employee bonuses companywide to its success and appointed Vic Gundotra, a senior Google executive, to lead it.

The value of Plus has only increased in the last year, as search advertising, Google’s main source of profits, has slowed. At the same time, advertising based on the kind of information gleaned from what people talk about, do and share online, rather than simply what they search for, has become more important.

Brand advertisers already target ads based on assumptions about broad categories, like women who watch sports. But the ads can be even more targeted when web companies know more about their users — say, that a particular female soccer fan is also a mother who likes thrillers and wants to buy a home.

“The database of affinity could be the holy grail for more effective brand advertising,” said Nate Elliott, an analyst at Forrester studying social media and marketing.

Google says the information it gains about people through Google Plus helps it create better products — like sending traffic updates to cellphones or knowing whether a search for “Hillary” refers to a family member or to the former secretary of state — as well as better ads.

“It’s about you showing up at Google and having a consistent experience across products so they feel like one product, and that makes your experiences with every Google product better,” Mr. Horowitz said.

Thanks to Plus, Google knows about people’s friendships on Gmail, the places they go on maps and how they spend their time on the more than two million websites in Google’s ad network. And it is gathering this information even though relatively few people use Plus as their social network. Plus has 29 million unique monthly users on its website and 41 million on smartphones, with some users overlapping, compared with Facebook’s 128 million users on its website and 108 million on phones, according to Nielsen.

The company has also pushed brands to join Plus, offering a powerful incentive in exchange — prime placement on the right-hand side of search results, with photos and promotional posts.