I've recently updated the social bookmarks plugin for Serendipity to version 0.44. There were problems with the feeds that ma.gnolia delivered, i.e. they changed the format of their RSS feeds. This is now fixed. They also now include preview thumbnails of webpages (bookmarked websites that is), which I added as an additional feature. Go and grab the latest version via Spartacus, CVS, or my download page. Feedback should be added to this S9Y forum thread.