The company that rules the social login has a leg up to dominate e-commerce, online advertising and a slew of other profit-oriented activities thanks to the data this activity can yield.

It's no small matter, then, that Facebook — which has long been the top platform for social sign-ins — is now showing signs of losing some of its market share.

So says a new survey by Janrain, which found that Facebook's overall market share of logins decreased 3 percent in the fourth quarter of 2014 after six months of consecutive growth. Meanwhile, Google's share of logins grew 6 percent, from 34 percent in the third quarter to 40 percent in the fourth.

It is a significant increase, Janrain claims — it is, in fact, the largest quarter-over-quarter increase in logins across all sites for any of the top identity providers in over two years.

LinkedIn's Big Jump

LinkedIn also showed momentum in the B2B space, pushing ahead of Google on these sites, from 29 percent in the third quarter to 35 percent in the fourth quarter .

It is a telling data point: Janrain concludes that B2B companies have come to realize the benefit of offering LinkedIn as a social login option. It is an "identity provider that passes over a data payload (with user permission, of course) that includes key professional data such as positions held and a verified email address."

Facebook and Google both lost a 3 percent share in this space.

Yahoo and LinkedIn held steady across all sites, with 4 percent and 3 percent of total social logins, respectively, while total Twitter logins decreased 2 percent, from 7 percent in the third quarter to 5 percent in the fourth quarter.

A Fluid Space

The big story, though, is Facebook's shifting position in this category – a position that could easily shift some more, in either direction.

For the moment, Facebook still dominates as the social login provider of choice in every vertical, Janrain noted, but is losing ground in the B2B, Music, and Consumer Brand categories for various reasons.

In general, Google is closing the gap for a number of reasons, according to Janrain's analysis:

Google's ongoing moves to unify identity across many highly-utilized services helps solidify its value and ubiquity for consumers—it’s a one-stop identity to manage Gmail, Play, Android, YouTube and Google+ accounts—so why wouldn’t consumers use it to manage accounts outside of the Google portfolio of products?"

This space is a highly fluid one though and in some cases, sign-in activity is more reflective of outside trends.

For example, among music-focused sites, Facebook lost 8 percent of logins while Google’s share jumped 12 percent, from 16 percent in the third quarter to 28 percent in the last quarter, Janrain figures show. Fluctuations in this vertical, however, are largely a result of the campaign-based nature of the industry and based around such events as concert tours, new singles and album releases.

A more reliable sector to follow would be consumer brand websites, which arguably serve the largest number of addressable customers. In this category, Google’s total share of social logins jumped 7 percent from 31 percent in the third quarter to 38 percent in the fourth quarter. Facebook's share decreased 3 percent, from 52 percent in the third quarter, to just under half of total logins at 49 percent in the fourth quarter.

Facebook's Comeback?

Facebook, though, is hardly peaking in terms of social sign-ins. Janrain predicts the dynamics in this category will shift again as Facebook migrates to its new app version introducing line-by-line controls and the anonymous login option.

"It’s also possible that their renewed focus on consumer data privacy will help Facebook regain some of the market share it lost this past quarter."

As Facebook prepares to launch the new version of its app on March 1, corporate customers are being asked to conduct an audit process that requires them to explain why they’ve chosen to collect certain pieces of customer information in their data payload, Janrain explains. Facebook can deny access to information "if it deems a company’s rationale insufficient, making it critical for customers to assess their customer data strategy going forward, and put plans in place for leveraging the data they collect from the people logging in on their sites."

Google Moves to Universal Option

Corporate users of the Google login, meanwhile, have their own issues to watch.

Google is preparing to deprecate their Google login on April 20 of this year, in favor of a universal Google+ option, Janrain notes. Companies using Google as a social login option must migrate to the Google+ identity provider prior to the sunset date to ensure that login doesn’t fail for your customers.

Battle for Conversion Rates

These trends matter greatly not just to Google and Facebook themselves but to the corporate companies that use these credentials for their own customers. Indeed, the conversions a social sign-in platform can deliver are closely watched.

As one example, last year when launched the version 3.0 app for the Android mobile operating system it noted that "Priceline Android users trend toward same-day hotel bookings, with a high booking rate in this category" and that "Android users who sign in with Google have a 71 percent higher conversion rate than those who simply book through the app as 'guests.'"