Just to add more information to these services - the websites allow you to build personal bookmark list that are then available online. They are of a special interest if you are a frequent traveler, and/or working often on computers you do not own.
Social Bookmarking means that you can share links with the community of similarly oriented people, and that you can see what others tell about the pages you bookmark. Personally I think that this feature is of just very little interest for the forum, because we already are in a community, and I doubt you can find a bigger freediving community anywhere on the Social Bookmarking websites or elsewhere on the web.
Furl offers a much more interesting feature - not only you can store your bookmarks online, but you can archive the pages or entire websites, so that you can access the original content even if the page expires or changes, or the server goes offline. Although also not too interesting for this forum (AFAIK, posts are being kept in the archive indefinitely), it may be very useful otherwise - websites pop up and die often faster than one expect, so keeping a personal copy of interesting information may prove priceless. It happened to me more than once that I bookmarked a website but when needed it, it was gone. Although there is always the possibility to dig the old content at
www.archive.org I have the feeling that their servers become saturated and slow, and that they often miss just the pages you are looking for.