Web servers have kept access logs from pretty much the inception of the web. They're not gathering any new information, tools like MixPanel are just organizing and presenting it more effectively.
The issue is cross-site analytics. MixPanel promises not to do it, but if multiple sites all used MixPanel it would be well within their capabilities to gather data on you as you move about their customers' websites.
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Web servers have kept access logs from pretty much the inception of the web. They're not gathering any new information, tools like MixPanel are just organizing and presenting it more effectively.
The issue is cross-site analytics. MixPanel promises not to do it, but if multiple sites all used MixPanel it would be well within their capabilities to gather data on you as you move about their customers' websites.