In the few weeks since we wrote “You Must be Mobile-Friendly”, Google has seriously upped their game. If your site is registered with Google’s Webmaster Tools, and is not mobile-friendly, you may have already received an email titled:

“Fix mobile useability issues found on http://yoursite.com/
To: webmaster of http://yoursite.com/
Google systems have tested 126 pages from your site and found that 100% of them have critical mobile usability errors. The errors on these 126 pages severely affect how mobile users are able to experience your website. These pages will not be seen as mobile-friendly by Google Search, and will therefore be displayed and ranked appropriately for smartphone users.”
This means that you absolutely must make your site mobile-friendly ASAP. If you need a reminder of exactly what that means – here it is straight from Google:

A page is eligible for the “mobile-friendly” label if it meets the following criteria as detected by Googlebot:
Avoids software that is not common on mobile devices, like Flash
• Uses text that is readable without zooming
• Sizes content to the screen so users don’t have to scroll horizontally or zoom
• Places links far enough apart so that the correct one can be easily tapped

Clearly, mobile compatibility is going to affect your search rankings, especially in searches done on mobile devices. The absence of the “mobile-friendly” label will eventually sink your listing below sites that can be easily read on tablets and phones.
Consider this: 80% of internet users own a smartphone and use it to search the internet. Mobile isn’t optional any more.