There are many ways to gauge what foreign language is popular. One way is to check how many books are at a bookstore. I went to a local Borders Bookstore and noted down the following. The numbers are the numbers of book shelves holding books for that language (self-study books, dictionaries, easy readers, etc):
(ancient) Greek: ?
Korean: less than 0.5
Russian: less than 1
Spanish: too many
Even if this were not in south Texas, I bet Spanish would still beat any other language. I'm surprised at the high number for French and relatively low number for Chinese, considering the fact that more high schools or junior high have both as foreign languages. But that's what the market is, or at least what the bookstore follows.
[Update] This posting has a follow-up in 2015: What language is popular? A revisit.