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I have a quite big website. I'm interested in knowing how big the overlap is between my users and the users of another website. Or in other terms what is the percentage of my users that also visited a specific other website.

I know this is a very hard-to-get answer, since e.g. cookies may only be accessed from you own domain.

What alternatives (services, technologies etc.) do I have for estimating or tracking this?

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up vote 0 down vote accepted

You can write JavaScript that determines whether or not your users have visited other sites by the time they have visited your site. The script would use the CSS properties of links to those sites to see whether or not they are visited. One example of a script that does this is Samy's CSS Knocker.

Some browsers view this as an intrusion into their users privacy and have put in place technical restrictions on querying the visited state of links using JavaScript. For example, these privacy measures were implemented in Gecko based browsers including Firefox. The StackExchange information security site has a question on bypassing these restrictions, but an answer states that doing so might be illegal.

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