Very good comparison (1) of cross-browser-bookmark-synchronization-services:
It seems, that Xmarks (2) has no real competitor concerning the cross-browser-bookmark-synchronization-abilities.
No other bookmark-sync-service than Xmarks is able to sync Bookmarks (and passwords) across IE, FF, Chrome, Mac OS X10, iPhone and iPad.
Additionally you quickly can share a Xmarks-folder (with subfolders) with your personal internet-bookmarks online as webpage and you can vote and see the average voting of other Xmarks-users for the quality of sites.
Further as an alternative to the Xmarks server you can use your own server for your bookmarks, and you also can sync the passwords across all your used browsers and computers, if you want.
Bookmark.com (3) has no Chrome-support and a bad reputation on (4), whereas the reputation of Xmarks is excellent on (5) nearly equal to the excellent reputation of AlternativeTo.net, see (6).
If you only use Firefox the best bookmark-solution is the firefox-addon Gmarks used together with the Firefox-Addon All-in-One Sidebar.
(1) Wikipedia: Comparison of browser synchronizers