Social networks like Google+, Twitter, or Facebook are similar. They do not only contain billions and billions of users and pages, they are also getting bigger all the time. Each day, more users join, and existing users add more content: they add more photos, share more links, and write more posts.
This is the major challenge in the engineering of social networks: to handle a system which is getting bigger all the time. An engineer of such a network is like a cook whose pots are constantly boiling over. He must add more pots each day, and keep the old still cooking.
Facebook was founded in 2004, and it was expanding ever since, gaining more users and more friends each day. Since it does not allow users to quit, and does not delete old content, it grows more and more each day. Twitter was founded in march 2006, and it was expanding ever since that time, too. Each day more users join, existing users gain more followers, and create more tweets. The latest and most advanced social network is Google+, which was started in July 2011 and combines the best features of Facebook and Twitter. It is growing at an ever accelerating rate, too.
The fundamental problem for the engineer is clear: a system which is getting bigger all the time is hard to handle, because it requires nearly unlimited scalability. If the engineers do not consider this from the start, it will be hard to build it later. Apparently, Google has the best prerequisites to master this challenge, it knows how to build scalable systems better than any other company. Some websites say Google is the King of scalability.
(The picture of the accelerating universe is from Wikipedia)