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However, I've recently been struggling and looking into some aspects of my site. I did a site search, and searched for a snippet of the meta description off some of my key pages. Sure enough 5-10 scraper sites have indexed the meta description I entered for the homepage/top level pages.
My site does not appear in the search results when searching for a snippet of most of my meta descriptions. Is this potentially causing me harm, as a search engine may be assigning my content to another site/page as mine don't appear. The only way I can think of fixing this is introducing the proposed meta description onto the page first, then mirroring into the meta tags once indexed.
It's different with the <title> tag which seems to be assigned against a source/site, but the meta descriptions (and probably keywords) are treated completley differently.
meta descriptions (and probably keywords) are treated completley differently
The meta keywords element is a total non-starter for ranking purposes on Google. The meta description often shows up in the snippet below the Title on the SERPs, but Google does not use the meta keywords tag as a relevance signal or ever display its content in search results.
You may want to continue using the meta keywords element for other purposes, such as an internal site search application or some directories - even Yahoo may use it just for spelling alternatives - but you can forget about any effect in Google Search.
The same logic applies as with the meta description - if the content doesn't display in the page, they don't want the page to rank for those words.
I appreciate your points on the meta keywords - it's a complete non starter from a google rankings perspective. I'd just like to delve slightly into what's stored in google's search index and what isn't, as if this is the case I'll modify my approach.
From a ranking perspective, I see lots of questions asked about the meta description, and what exactly it does for a site. I've been under the impression that it's not related to rankings from a google perspective, but religiously I repeat my main keyphrases / words for the page to a) highlight on a serp, and b) in case it's still taken into account in the ranking algorithm behind the scenes.
After finding the meta descriptions aren't in the main google index and are readily duplicated on other sites, it just threw me slightly and made me think about them :)
But as you've seen, meta descriptions are not directly searchable - and neither are keywords. They are "indexed" in the strictest meaning of the term, but they are not tagged to be searchable.