It may seem like Facebook and Twitter are all we hear.  But that’s just a piece of the huge social networking puzzle.  Bebo seems to be one to watch: keep track of friends on other social sites in one place.  I’m giving that a try and also putting together a social network using Ning as a platform… really incredibly simple and powerful.  Here’s Adam Ostrow‘s take on the social scene in The Fastest Growing Social Sites:

• Each month (and often more frequently than that) we take a look at trends in social networking, by the numbers. Recently, we’ve focused on Twitter’s astronomical growth – now up better than 2,500 percent in one year – and Facebook’s climb to become top social network over MySpace ().

  However, this month there are a few other trends within the numbers – provided by Nielsen Online – that highlight a few other storylines within the social media space, as well as continue to put meat behind the trends we’ve been covering over the past few months.

  Here are some notable developments from March:

  Ning, which recently announced that more than one million social networks have been creating using its service, is currently the 2nd fastest growing social networking property. It’s traffic is up 283 percent year-over-year, and it now reaches a total of 5.6 million people in the US.

  Bebo, which we recently reported saw a one month surge of nearly 50 percent on the heels of a redesign and AIM integration, has grown 148 percent in the past year, and now reaches more than 6.1 million people in the US.

  This is significant because Bebo is known mostly for being big overseas, but insignificant in the US. The next few months will tell us if this was a one-time pop thanks to AIM integration or a continued trend thanks to some innovative things Bebo is doing with Socialthing.

  LinkedIn () continues to be one of the quickest growing social sites and has more than doubled its size in the past year. It now reaches better than 15.8 million people in the US, ranking it 3rd in total size behind Facebook () and MySpace. It’s no doubt being buoyed by the weak economy and high unemployment as people turn to the site and their connections to find work.

  As for “the big three,” the story remains much the same:

  Twitter () is now growing at a mind-boggling 2,565 percent. In total, it reached more than 13 million people in the US during the month – and that’s just on its website (i.e. – not counting clients like TweetDeck () or Seesmic Desktop ()). Next month will likely see this total continue to swell, thanks to huge mainstream media events like Ashton Kutcher versus CNN and Oprah jumping onto Twitter.

  Facebook continues to extend its lead over MySpace. In March, it saw 69.1 million visitors, versus 55.9 million for the News Corp-owned social networking site. In fact, MySpace was the only social networking site measured in the Nielsen survey that saw its traffic decline year-over-year.

  So, while not much changed at the top of the social media landscape in March, there are certainly a few new storylines emerging that will be worth watching going forward. The full report – sorted by growth rate – is embedded below:

  Image courtesy of iStockphoto, alexsl