Tosh - it is almost certainly NOT 'Malware' - it is 'Nuisanceware'. Many 'free' sites will be paid by 888 or others to have their pages 'pop up' when you visit the host site. Snopes is/was one, many 'free gifs' sites do as well - it is how some sites 'fund' their content (and make money!), just like PPRune does with all the adverts. I have adblock plus and it does not stop all the pop-ups. If malwarebytes has come up clean I reckon you are ok, but don't drop your guard!