Try this:
\.example\.com\/forum\/popular\/buzz\?tab\=popular
"=" is significant to regular expression matching for assignment and needs to be quoted. "?" is significant as well and although you don't strictly need to quote it with a "\" to make it work, doing so may improve performance slightly. If it still doesn't match try toggling the checkbox "URL is a regular expression" on the custom site/app object.
Edit: clarified what ? matches based on Bob's post
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