Q:

the means of phrase "take this thing for granted"

what does this mean ?

So we take this Internet thing for granted, but it's a pretty fragile physical system.

A:

It is about our (author used we) perception of Internet, about the nature of emergence of Internet .
Author tell, that there is a common opinion: Internet is the same thing as water, stone, earth and air, something given to us, or something available to us regardless of our activity; something, that will exist for ages in any circumstances and conditions. Then author told, that such point of view is wrong, and actually, the Internet is a difficult and sometimes brittle system.
PS: actually, Internet is not such brittle system as porcelain or autumn leaf or schoolgirl's self-esteem. Most likely, Internet is a dreadnought, or Human nervous system, or Indian rail network — enough reliable and self-regulating system.

