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).
(1) http://en.wikipedia.org/wiki/Comparison_of_browser_synchronizers
(2) XMarks
(3) http://alternativeto.net/software/bookmarkcom/
(4) http://www.mywot.com/en/scorecard/bookmark.com
(5) http://www.mywot.com/en/scorecard/xmarks.com
(6) http://www.mywot.com/en/scorecard/alternativeto.net

this is just one of many reasons, Xmarks will be shutting down its services.

Tried both Weave as XMarks, and Mozilla won me with their easy, clean and simple setup. Pro for XMarks is that it works throughout multiple browsers where Weave is Mozilla browsers only. Since I work Firefox-only the easy and full featured sync of Weave beats XMarks hands down for me nonetheless.