4.3 Billion is the best guess.
It’s often forgotten by us “First World” people that there are many – the majority of the worlds population in fact – That have never “gone online”. I’ve wanted to repost a map I saw in National Geographic showing a world map done in black with every IP address represented by a light. It is here on an interactive page on National Geographics website. The world is mostly without internet, except for all of the developed areas.
This info-graphic from http://www.visualcapitalist.com/ brings home the idea. The internet is not ubiquitous.