This fundamental assertion is wrong. The "company's currency" is not simply about the number of users. What it is really about is the revenue that can be generated via users. Having lots of users but no way to monazite them (Twitter) is useless to them.

In both cases, Google has more users and generates more revenue per user than Facebook. Facebook's stock got beaten up when investors noticed the revenue per user was lower than Google and was going down instead of up on mobile.

While Facebook is a potential threat to Google if Facebook figures out how to increase its effectiveness, Amazon (which is also mentioned in the article) is Google's bigger, more immediate threat.

Most of Google's advertising revenues come from people searching for things they want to buy. Amazon already has a search engine for shopping, a large inventory or items, an affiliate network of other vendors willing to sell you stuff you're looking for, and shopping recommendations to lead you to related products.

If enough people go directly to Amazon for their search shopping needs, this will destroy most of Google's advertising revenue model.