Firstly, a definition: a "pop" word is one which occurs in the most-used 87,000 according to the British National Corpus. All rows in this article are pop words, which naturally include initialisms.
"Exotic Varieties of Four-Square,"
Word Ways: Vol. 41
, Article 9.
Available at: http://digitalcommons.butler.edu/wordways/vol41/iss1/9