One account, all of [yours for] Google

Google's login page says "one account; all of Google." I would read it as "one account, all of yours for Google." That's what happens when a single company runs multiple online services, and all of it is accessed via single sign-on.

It is well-known that Google knows a lot about each of its users (CNBC report here). It even records every voice command one makes, as reported by The Guardian.

Google itself admits these in their privacy policy. Of course, Google appears to be much transparent than many other online services. They have set-up dashboard(s) where one can see what information regarding (s)he is collected, where one can limit and delete this collection. But that isn't sufficient.

First, we can never be sure such dashboards cover everything they know about you. But we won't know even if it were some other company. So this alone isn't a valid argument against Google.

What really matters is, Google collects its users' data by default, and this data is shared across all the services it owns. Collection of private information shouldn't be enabled by default, unless it is crucial for providing the service. Most of the data Google (and similar) companies collect are in no way necessary to provide basic functionality. Although they argue this collection is to enhance user experience, it is actually used by them to train their systems (and some companies--I hope not Google--sell such information). Some free (as in freedom) software projects also collect user experience data, but it is only after gaining an exclusive user consent, and the data is anonymized (to the extend I've seen).

Google and similar companies should stop collecting extensive user information. And users should never let them collect such information.

In any case, it's a bad idea to be a registered user of a company that owns different online services that are interconnected. You shouldn't own a mail account, a remote storage account, a social account, and a search account with the same company.

When Google (or any other company) alone becomes a synonym for the Internet, that would be the end of online privacy.

You can use one or two services from the same company, that too after taking appropriate privacy measures. You can use many, if they can be accessed without identifying yourself, and without proprietary software.