it’s not ,sorry,friend of my @ work told me his sure, but I’ve just checked the code and run some tests to be sure and came to conclusion they are not using bp or wp.
when you’re not sure if the site is using wp or bp you can test it by :
if you’re logged in and try to post something(comment or activity update), before you post it , open the same link in another tab ,then logout there and then go back to the first tab and hit post,it will show you the standard wp error ”are you sure you want to do this”
another one is : you can check out the upload directory which in most cases is wp-content/uploads … (avtars, etz.)
So Mashable is definitely not using WP OR BP.
sorry