The Guardian newspaper is certainly one of the more experimental UK newspapers, as we reported back in October when it launched its N0tice project, something which could shake up the local newspaper market. And, of course, it also recently unveiled a US-focused news website as it seeks to drive its international readership, and as with many publications, it has a dedicated iPad app too.

However, back in September the Guardian launched a Facebook app too, essentially letting Facebook’s 800m+ users read the guardian’s articles online without actually leaving the social network.

A little over two months on, and it seems that its Facebook app has proved pretty popular, as it has now been installed by more than 4m people. This has apparently generated, on average, almost a million extra page impressions each day for the publication. And it seems this may be helping the Guardian tap into a much younger readership, as more than half  (56.7%) of the app’s users are 24 and under. “This is traditionally a very hard-to-reach demographic for news organizations,” says Andrew Miller, CEO of Guardian Media Group.

Delving a little deeper into the stats, it seems that well over half-a-million (16.7%) of the people who have installed the app are 17 and under. “As well as increasing traffic, the app is making our journalism visible to new audiences”, says Miller. “The Facebook app is one of a number of successful launches by the Guardian in recent months as our digital-first strategy gains momentum. We’re delighted with the results.”

The Independent newspaper announced its own Facebook integration around the same time as the Guardian, which followed in the wake of a heap of new announcements from the social network at its f8 Conference.

Whilst the Independent has fewer numbers to boast of than the Guardian, more than one million monthly active users are connecting their Facebook accounts with the social news functionality on its website. The integration has helped promote older articles that have gone viral through social distribution channels, and many of the “most shared” and “most viewed” articles on the site have been from the late 1990s, as noted by Tim Bradshaw from The FT a couple of weeks back.

It’s probably also worth noting that Yahoo! News, which built a deep Open Graph integration into its site, now has more than 10m people signed up to the Facebook reading experience, seeing a 600% rise in traffic coming from the social network, whilst the Washington Post has taken on 3.5m more active users through Facebook integration.

Around this time last year, we reported on an experimental project called The Social Guardian, which was built using The Guardian’s API. Logging in to the site with your Twitter account, you can see what other Twitter users are reading using the service, refreshing in real-time as they load new articles. A feed updates the page with the latest content from The Guardian website, and it’s easy to see what others are reading.

In terms of the latest social integrations, this is all great news (no pun intended) for the publications involved. But it’s probably even better news for Facebook, as it looks to grow its user-base and become a sort of ‘Internet within the Internet’. News of the success of these publications won’t go ignored, and many more publications will no doubt jump on board in the months and years that follow.

In January this year, we questioned whether Facebook could hit the magic billion-user mark in 2011. It may fall just short of that milestone this year, but at the rate it’s going it won’t be too far off.