Maybe you are all high and mighty and don't use "just some social networks", but they have definitely changed the world, in HUGE ways. I would say facebook's global permeation (circa 2009) is probably the most world-changing thing since global consumer internet availability (approx 1998?).
Just the other day, we on HN watched as a hotel business most likely crumbled because of worldwide attention on such social media [1]. Similarly, entire businesses are livelihoods are being built upon being visible on facebook. News travels like wildfire, opinions can make CEOs step down, a viral video can turn a beginner musician into a contemporary sensation, the list is endless.
How does one deny that these things have changed the world compared to before? They may not be all good changes, or any of them good changes, but they have changed the world, in huge ways, and we had best learn to accept it and take it into account in our lives.
[1] https://news.ycombinator.com/item?id=8131162
Edited to actually add link.