Business networking site LinkedIn has acquired Rapportive, a Gmail add-on that provides information about your social contacts as you e-mail them. The deal was reportedly already in place by Dec. 8, but Rapportive confirmed the acquisition on Wednesday on its company blog.

Over the last two years, Rapportive has become an essential product for folks all around the world, said Rahul Vohra, Rapportive's CEO. When rumours of our acquisition surfaced last week, many asked what was going to happen to the product. Well, we have fantastic news: At LinkedIn, we will support Rapportive, and we will continue to build beautiful products that make you brilliant with people.

Vohra explained that Rapportive got to know LinkedIn over time, and found that there was a high degree of overlap between the two companies' visions.

We found a high-caliber and extremely driven team, Vohra said. We found, crucially, a 'members-first' company: everything that LinkedIn does is about making its members more successful. In business, partnership is dating - and we went on a lot of dates with LinkedIn. Slowly, but surely, we fell in love.

Later in the post, Vohra was much more animated and fired up about the deal:

We are completely beside ourselves with excitement, Vohra said. I have so much I want to tell you, and so much I want to show you. But I need to restrain myself - there'll be time for that later. For now, let me leave you with one last thought.

A company, at its core, is a set of beliefs united by a vision. When we founded Rapportive, we had one simple belief: we would build software that you don't have to remember to use. Our software would be an intrinsic part of the tools you use every day. It would be there when you want it, and out of the way when you don't... When you build software into the very fabric of the world around us, when you remove friction from the things that people want to do - something magical happens. You enable people to change their own behaviour. You empower people to become better at what they do. And if you get enough people to do that, you might just change the world.

LinkedIn and Rapportive both declined to comment on the financial terms of the deal, but sources told AllThingsD at the beginning of the month that LinkedIn offered the browser plug-in a figure in the low teens in millions of dollars.

Rapportive, which is still available over Gmail, adds an e-mailer's social networking accounts, including their Facebook, LinkedIn and Twitter accounts, and overlays the information over open messages and e-mail drafts. If you follow the user on Twitter, you could see their most recent tweets, or if you haven't connected with them on LinkedIn yet, you could easily visit their profile and change that.

I wanted to build a product to make things easier, Vohra said. In Gmail we show you what people look like, their tweets, what they are into, their social networks, etc. We add a lot of context to Gmail.

Rapportive has never released how many people use the add-on, but Vohra said users currently look at over 65 million contacts per month, and the company has grown its user base many times over, despite only having six employees.

I am now following my passions, Vohra said. I just love building experiences that delight users.

Vohra founded Rapportive in January 2010 with two other computer science graduates from the University of Cambridge in the UK, Martin Kleppmann and Sam Stokes. The small company took their talents to San Francisco in 2010 because they found it would be an easier startup atmosphere than Great Britain.

In one week we accomplished what would have taken us a month back in Cambridge, Vohra said.

The company was accepted to Y Combinator, the startup accelerator based in Mountain View, Calif., based on the fact that their company had 10,000 users sign up on the first day thanks to an accidental leak to the press.

We applied, accidentally launched, got into YC, and then raised our seed funding round during this period, Vohra said.

Rapportive raised $1 million in August 2010 from a group of angel investors, including Paul Buchheit, the founder of FriendFeed and the creator of Gmail. In May 2011, Google launched a potential competitor to Rapportive, a people widget that integrated Buzz posts, shared documents and calendar events directly into Gmail to similarly view relevant information in context.

We're very flattered by how similar the widget is to Rapportive, Vohra said. In fact, some of the design details have been copied directly, from the new position of the 'print' and 'new window' icons, through to how the widget remains onscreen as you scroll.

But instead of beating its competition, Rapportive joined it instead, integrating Gmail's people widget directly into the Rapportive experience.

The press sensationalized it, saying that Rapportive would die, Vohra said. But, of course, we didn't die -- competition very rarely kills start-ups.

Rapportive will be a tremendous help to LinkedIn, and will likely aid the business network by providing organized user information for job seekers, employers, entrepreneurs and other business-minded individuals using the site. In this marriage, Rapportive can share information from a user's social network -- so long as there's permission -- which would make the lives of millions of job recruiters using LinkedIn significantly easier.

The main reason that companies are using LinkedIn is to find passive job candidates, said Kay Luo, LinkedIn's director of corporate communications. Another reason why companies are using LinkedIn is because referrals from their employees are highly valued because they typically have a higher success rate -- hence the popular 'employee referral bonuses.' LinkedIn helps companies leverage the networks of their employees.

Rapportive will not drastically change how LinkedIn performs, but having more information about job candidates makes recruiters make better, more well-informed decisions. After all, most hiring decisions nowadays require recruiters check a candidate's social media activities, which give an organization an idea of how professional that person is in the digital world.