When the writer wrote that Twitter loves open source he wasn't exaggerating. He was quoting from Twitter's About Open-Source page. There, Twitter states that, "Twitter is built on open-source software-here are the projects we have released or contribute to."
It's quite a list of C++. Java, Ruby, Ruby on Rails, and Scala programs. If you dig deeper, by looking at Twitter's job postings, you'll see Twitter also makes great use of the standard LAMP (Linux, Apache, MySQL, and Perl/PHP/Python) stack. In particular, being able to demonstrate that you've been open-source contributor is a big plus for many Twitter jobs.
There are also some open-source programs that everyone in the social networking world uses, but are far less well-known in the broader open-source universe. The most important of these is Memcached.
Memcached was created by LiveJournal engineers. It's a high-speed, distributed memory object caching system, that's meant to handle the kind of massive database loads that come with real-time Web applications like social networking. Besides the social networks, for example, Memcached is an important part of Craigslist, YouTube and Wikipedia's IT infrastructure.
This program caches creates a global cache of all the millions of data requests that social networks and other popular Web sites generate. The end-result is that users get to see updates in close to real time, while the database servers get to not melt down into piles of slag from all the demands on them.
Does it begin to sound to you like not just the social networks, but much of the so-called Web 2.0 Internet relies on open-source and Linux? It should, because it does. Without LAMP and the new open-source technologies that go along with it, the 21st century Internet simply wouldn't exist.