According to W3, WordPress now powers more than 22% of the web. It was the fastest growing CMS in 2013, and that trend has showed no signs of stopping. We conducted a study of over 48 million sites using the SEO service Lipperhey and found that, of the sites with CMS’s that the SEO tool was able to identify, WordPress powers 68% of them.

If you needed proof that SEO has gone mainstream, this study found that 37 million of those sites powered by WordPress had SEO plugins installed. More astoundingly, between 40 and 48 percent of those sites were using either Yoast or All-In-One SEO Pack, at least in the United States and Europe. And while the number of installs of these two plugins are almost neck and neck in the West, All-In-One SEO Pack completely dominates the Asian market.

But these are just the two most popular plugins—Ultimate SEO, Platinum SEO, and SEO Booster are just a few of the myriad available options. Lipperhey ran their on-page SEO scoring system against the 25 million WordPress sites using these five SEO plugins to see which ones scored the best. The radar graph below shows how they scored each site against these five on-page SEO factors:

• % NOT KEYWORD STUFFING
  • This seemed like an obvious thing to check for.
• % UNIQUE PAGE TITLES
  • While this is no guarantee of good rankings, websites that are optimized tend to have a higher incidence of unique page titles, whereas sites that are fresh out of the box tend to use a generic one.
• % USING H1 TAGS
  • As above but again, use of h1 tags suggest a deliberate efforts to optimize a site.
• % PAGES WITH UNIQUE META DESCRIPTION
  • Meta descriptions may be fairly unimportant in the grand scheme of things, but SEOs know that it’s good practice to optimize meta descriptions, if only to improve the appearance in social networks.
• % PAGES OPTIMIZED
  • This is a default Lipperhey calculation that tries to identify if a page is targeting any keyword in particular by page, a metric that we found to be very instructive in our initial tests.

Fortunately, none of the plugins were used to generate blatant spam sites, with a low incidence of keyword stuffing on all installs. Each plugin also did a good job of ensuring that every webpage had a unique page title. The Yoast plugin claimed the highest percentage of sites using headline tags, but as with SEO Booster and All-In-One SEO Pack, meta descriptions consistently needed to be improved. Despite only claiming a 2% market share worldwide, Platinum SEO Pack had the highest percentage of top scores on these five factors.

As a result of comparatively lower scores on headline and meta description tags, the percent of pages optimized was the lowest on SEO Booster, Ultimate SEO, and Yoast. But there’s no cause for alarm: it actually shows just how much more popular the Yoast and All-In-One SEO Pack plugins are, with both of them claiming the highest count of top scoring sites for SEO.

Although Platinum SEO Pack had the highest percentage of high-scoring installs, suggesting an attentive and knowledgeable user base, that statistic was completely overshadowed by the market dominance of Yoast and All-In-One SEO Pack. With each plugin claiming over 20% market share on WordPress sites, Lipperhey’s score data suggests that both plugins have become the de facto installs for WordPress SEO-conscious webmasters—even if it means, in some cases, not taking their SEO work any further.

All that being said, this data doesn’t necessarily indicate which is the best or most effective of these plugins—it’s actually more indicative of how they’re being used, or how good people are at using them. Just because people using Platinum SEO Pack were able to produce more optimized pages, for instance, doesn’t mean that the plugin’s design has somehow propelled them to this achievement. It’s more likely that Platinum has a highly skilled fan base, which is a good thing, but an unmistakably different claim than that of superior design.

By the same token, the most popular or common plugins aren’t necessarily the ones that will earn your site the highest rankings. In fact, according to Lipperhey’s data, sites optimized by Yoast had some of the lowest performance rankings. But plugins with high downloads and low performance suggest a lot of unused installs—Yoast’s popularity might end up pulling down the the average score in this study because, since more people have heard about it, it can be installed by default even if they end up not using it or, when they do, using it poorly.

Regardless, broad popularity and still relatively high scores indicate that these plugins are intuitive and effective, making it useful to both SEO experts and novices.

Still, popularity has its downsides, as well. Because of how common WordPress-as-CMS is now, the platform does attract a lot of attention from hackers – and by extension their plugins. The web security firm Sucuri recently discovered vulnerabilities in All-In-One SEO Pack’s software, meaning that websites that use the plugin could suffer from attacks that inject malicious code and link spam which can drive down a site’s ranking.

Although the team behind the popular plugin quickly corrected these flaws with a patch, this situation could cause a case of buyer-beware (although it’s free) among consumers. Just as an SEO plugin is no guarantee of better rankings, one can’t just “set-it and forget it,” either. Regular updates to the latest version of WordPress and the plugin are required.

Furthermore, I do think that some of the other plugins could also improve on this test. Some things the study fails to address are the exact size and function of the sites that we analyzed – from brochure sites to ecommerce, or the difference between b2b and bsc. For example, the same test could be refined for a cluster of sites in the same sector which are nominally targeting the same keywords. For those looking for a more rigorous analysis of quality, I would highly recommend reading this summary of the top line differences between the plugins and this excellent post, which extensively compares the more advanced features found in Yoast and All In One SEO Pack (), written by Dan Shure.

In any case, the data on these SEO plugins is fascinating for its own sake. As people become more and more familiar with SEO and maximizing public exposure to their websites, the popularity of specific plugins serves as a mirror of the public’s understanding of SEO in general. Given that 77% of the 37 million sites surveyed powered by WordPress containing SEO plugins, there’s no doubt that the concept is now broadly understood among webmasters and businesses alike.

Though it’s hard to draw definitive conclusions from this data about which plugins are the most effective, secure, or easy to use, it does represent an excellent starting point for anyone interested in doing SEO themselves and gives developers a sense of the hurdles a new SEO plugin will face to gain traction in the market.