I'm using both on a site and getting very different numbers from each. Why is this?
The discrepancy is also mentioned in a Quora answer (Which is better, Facebook Insights or Google Analytics?)
Footnote: if you decide to use both, do not report them side-by-side, and never expect them to match. Trying to explain the differences will drive you mad.
Could someone explain?
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This problem is quite common, and very hard to explain to clients why numbers do not reconcile amongst different analytics platforms.
Firstly, I believe that because there are remote connections to google or facebook some user sessions will get lost (What happens when they hit stop on the Browser page before the .js downloads for instance).
Secondly I believe ad blocking software may stop the file from being downloaded therefore the session is not captured.
Most hosting providers will have their own analytics platform with your hosting package. This is what I rely on as a true indicator for actual page views etc. These are usually generated directly from your web server logs so they are more accurate. Sadly I've never seen one of these packages have as many features as google or facebook.
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In addition to the notes above, I also wanted to mention Google samples data when there are large volumes & dimensions. This may be a contributing factor.
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There are tons of possible reasons. They might identify returning visitors in a different way or users might block scripts from a specific domain (e.g. *.facebook.com but not *.google.com). In general, ignore the discrepancy. Just pick one solution and use it. You'll always have visitors blocking all such scripts or just one or two specific trackers. The only (almost) 100% accurate way to do it, would be using local scripts, but even those could be blocked. You could as well look at open source solutions such as Piwik
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Facebook reports on clicks and Analytics reports on pageviews.
The amount of pageviews might be less than the amount of clicks for a number of reasons:
This seems to be a big problem with Facebook ads. I run a number of campaigns with facebook and I only see 30-50% of the reported traffic actually make it to the site. I cant believe this is due to only the first two reasons.
I have gone into more details on my blog http://www.bradtollefsen.com/facebook-ads-adding/
Different web analytics products use diferent methods to track data on the site. These differences between them is the reason why is hard do do a side-by-side comparison.
On the two links bellow you can find more info about that:
Why does Google Analytics report different values than some other web analytics solutions?
Using Google Analytics & Facebook Domain Insights to Track Social Actions on Your Website