Shojo Prefer Shonen Jump
According to a recent Japanese survey, the top manga magazine for girls in Japan was actually Weekly Shonen Jump. Magazines actually targeted towards girls all ranked lower. The reason why Shonen Jump ranked highest was because of manga that are popular with both boys & girls, specifically listing One Piece, Death Note, & the Prince of Tennis. NANA was said to be the next most popular manga for girls (& the highest ranking shojo title), & it was because of it that Monthly Cookie came in as the #2 magazine.
That's not to say that all other shojo titles aren't popular. Also just recently released was a list of the top selling manga for the past week. "Ouran High School Host Club" #10 (aka, the anime with the other Haruhi) ranked in the top 5 with major shonen juggernauts Bleach, Naruto, Detective Conan, & Eyeshield 21. Bleach #27 took the top spot, while Naruto #37 took the second spot.
Sources: http://www.animenewsnetwork.com/news/2007-04-11/japanese-comic-ranking
http://www.animenewsnetwork.com/news/2007-04-10/oricon-no.1-manga-mag-for-japanese-girls-is-cite-shonen-jump/cite