How do you qualify as "mobile-friendly"? Google cites the following criteria:
- Avoid software not common on mobile devices (i.e. Flash)
- Use text size that is readable without resizing
- Scale content to the screen so user doesn't have to pinch or zoom
- Leave enough space between links and tappable objects
In addition to Google's guidelines above, make sure you're not committing any of the 6 deadly sins that could cripple your mobile SEO:
- Faulty redirects
- Smartphone-only errors
- Unplayable videos
- App download interstitials
- Irrelevant cross-linking
- Slow loading pages
Responsive or adaptive?
It doesn't matter if you're responsive or adaptive (or a hybrid of both), though Google in the past has recommended responsive design, with a preference for a single URL structure. Forrester analyst Peter Sheldon has observed that mobile sites using a separate mobile domain (such as m.site) do not appear to pass Google's mobile-friendliness test.
Is your site up-to-snuff? Take Google's Mobile-Friendly Test and find out, or consult your Mobile Usability Report in Google Webmaster Tools.