LinkedIn announced earlier this month that the social network has reached 100 million users worldwide, and gaining 1 million new sign-ups a week. 44 million of those users are in U.S., but how active are those users? Let’s see if we can break it down.

According to reports, as of January 2011, LinkedIn only received .51 percent of all social networking site visits by U.S. social networking consumers. Facebook, of course, came in at #1 with 90 percent of all visits. But it looks like Myspace, Twitter, Tagged, myYearbook and MyLife all had more active users than LinkedIn. However, comScore reported in December that LinkedIn’s U.S. numbers were actually ahead of Twitter - 26.6 million unique visitors while Twitter only had 23.6 million that month.

According to The Media Audit, LinkedIn only had 11.5 percent of U.S. users logged-in to the social network in that month. In February, Arbitron/Edison Research had found that only 8 percent of U.S. consumers used the professional social network. Considering that few number of kids and those in their early teens who use LinkedIn (presumably because they do not have jobs or need to network), those low numbers are expected.

Even less surprising is that Wedbush Securities reported in November 2010 that fewer social networking consumers were likely to use LinkedIn than Facebook and Twitter, and even less would use LinkedIn on a daily basis. However, Sheraton Hotels & Resorts reported in June 2010 that LinkedIn users weren’t logging-in to the site as regularly as Twitter and Myspace users, but were checking the site more than Facebook users.

But LinkedIn continues to argue that their user base is a perfect audience for social media marketing. The company boasted 18 million users who were part of groups. In fact, they report that their groups are gaining 1.5 million users and 1.2 million comments and posts per week.

LinkedIn seems to be coming up with new features on a regular basis, creating social services for a younger crowd and integrating with Hotmail, so it’ll be interesting to see how it continues to compete with other social networks. LinkedIn still finds itself in the in-crowd worldwide and with it’s steady growth, we’ll see how the social network will rank by the end of the year.

How regularly do you login to LinkedIn?