Facebook is leading the charge for social media sites in the #4 slot for unique visitors, according to media metric ranker comScore. Facebook sits up there with the big ones like Google, Yahoo and Microsoft with just over 153 million unique visitors in January.

You have to scan down a bit to find LinkedIn at #36 with 29 million unique visitors. It’s seen a recent surge in popularity lately, as we noted earlier, thanks to more businesses using it to network. Yelp brings up the back at #45, still keeping a strong showing and hanging in the top 50 with 26 million unique vistitors.

Facebook taking the lead in social networks isn’t surprising, but what is noteworthy is the fact that it finally made it up to the top ten in the advertising-focused metrics, reaching 72.3% of the 212 million Internet users in the U.S. This is good news for the media giant, who can now show their eye-catching appeal to investors and marketers.

Not being in the top 50 for views doesn’t mean you’re out of the game. YouTube ranks at #26 on the ad-based metric chart, reaching 54.1% of people on the Internet. Meebo (#37) and Myspace (#47) also are good eye-grabbers for ads, reaching 36.8% and 21.2% of you out there.

ComScore says that January is a big month for taxes and vacationing. This gives a big boost to web-based tax services and travel websites, which was good news for Yelp - which is proving to be a mainstay for people who use the website to search for local attractions.