We're getting a lot of questions about the upcoming Google mobile algorithm update, and so I wanted to start a discussion that covers what we know at this point (or, at least, what we think we know). If you have information that contradicts this or expands on it, please feel free to share it in the comments. This is a developing situation.
1. What is the mobile update?
On February 26th, Google announced that they would start factoring in mobile-friendliness as a ranking signal. The official announcement is here. Of note, "This change will affect mobile searches in all languages worldwide and will have a significant impact in our search results."
2. When will the update happen?
In an unprecedented move, Google announced that the algorithm update will begin on April 21st. Keep in mind that the roll-out could take days or weeks.
3. Will this affect my desktop rankings?
As best we know - no. Mobile-friendliness will only impact mobile rankings. This is important, because it suggests that desktop and mobile rankings, which are currently similar, will diverge. In other words, even though desktop and mobile SERPs look very different, if a site is #1 on desktop, it's currently likely to be #1 on mobile. After April 21st, this may no longer be the case.
4. Is this a boost or a demotion?
This isn't clear, but practically it doesn't matter that much and the difference can be very difficult to measure. If everyone gets moved to the front of the line except you, you're still at the back of the line. Google has implied that this isn't a Capital-P Penalty in the sense we usually mean it. Most likely, the mobile update is coded as a ranking boost.
5. Is this a domain- or page-based update?
At SMX West, Google's Gary Ilyes clarified that the update would operate on the page level. Any mobile-friendly page can benefit from the update, and an entire site won't be demoted simply because a few pages aren't mobile friendly.
6. Is mobile-friendly on a scale or is it all-or-none?
For now, Google seems to be suggesting that a page is either mobile-friendly or not. Either you make the cut or you don't. Over time, this may evolve, but expect the April 21st launch to be all-or-none.
7. How can I tell if my site/page is mobile-friendly?
Google has provided a mobile-friendly testing tool, and pages that are mobile-friendly should currently show the "Mobile-friendly" label on mobile searches (this does not appear on desktop searches). Some SEOs are saying that different tools/tests are showing different results, and it appears that the mobile-friendly designation has a number of moving parts.
8. How often will mobile data refresh?
Gary also suggested (and my apologies for potentially confusing people on Twitter) that this data will be updated in real-time. Hopefully, that means we won't have to worry about Penguin-style updates that take months to happen. If a page or site becomes mobile-friendly, it should benefit fairly quickly.
We're actively working to re-engineer the MozCast Project for mobile rankings and have begun collecting data. We will publish that data as soon as possible after April 21st (assuming it;s useful and that Google sticks to this date). We're also tracking the presence of the "Mobile-friendly" tag. Currently (as of 3/25), across 10,000 page-1 mobile results, about 63% of URLs are labeled as "Mobile-friendly". This is a surprisingly large number (to me, at least) - we'll see how it changes over time.