Twitter and Facebook usually get all the hype due their size and ubiquity, but this week, the spotlight's on professional networking service LinkedIn.

Earlier this week, the site announced it has reached the milestone of 70 million users, adding 10 million members since February.  Though that's still a fraction of Facebook's 400 million-strong user base, Leena Rao notes on TechCrunch that there's "a tremendous amount of potential to use the data the network has on its side to add additional functionality to both users and companies".  In fact, LinkedIn looks to be taking its cues from recent changes Facebook has implemented, giving its Groups function a make-over.

LinkedIn promises that the changes it is making will advance the "democratization of discussions", by allowing group members to comment on and "like" conversations and highlighting the individuals the site identifies to be "top influencers" responsible for generating the most activity.  LinkedIn is taking cues from Twitter, too, with a new 'follow' function - users can select individual group members to follow and receive an email alert every time that individual makes a contribution to the group.  By judging influence based on reactions from other site users, LinkedIn has effectively set up a check on its Groups being hijacked by bombastic super-users.

Lest these changes make you suspect that LinkedIn is abandoning its professional credentials in favor of hopping in the social media sandbox, there are plenty of indicators showing that LinkedIn is staying true to its professional/B2B roots.  In March, the site made its first BlackBerry app available, with Storm and Pearl-comptaible versions released earlier this month.  Again following the blueprint assembled by Facebook and Twitter, LinkedIn's decision to offer a mobile app enabled its users to enjoy uninterrupted (except for when driving, of course) access to their professional networks - absolutely necessary for the demands and needs of modern business.

Additionally, Harry Gold lists eight new media advertising opportunities LinkedIn offers, such as upgraded company profiles, targeted PPC DirectAds, and Answers category sponsorship, that enable companies to "accurately target professionals by job titles, verticals, or even horizontals".  Given the site's fundamental networking component, LinkedIn's ability to parse its users' data and activity could add significant value.  Third behind Facebook ($11.5 bn) and Twitter ($1.4bn), LinkedIn has a SharesPost suggested value of $1.3 bn.

LinkedIn may be third-in-line to the social media throne - but it has clearly staked its claim on providing a personally fulfilling and commercially useful experience for online professional networking.

Straight from the source: watch the video below for a quick introduction to LinkedIn Groups' new features.