SAN FRANCISCO — Google Plus, the company’s social network, is like a ghost town. Want to see your old roommate’s baby or post your vacation status? Chances are, you’ll use Facebook instead.

But Google isn’t worried. Google Plus may not be much of a competitor to Facebook as a social network, but it 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.

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.