My browser bookmarks always got into a mess quickly. I use different computers, I'm not very organized and they don't have any timeline embedded. I wanted bookmarks in the cloud, accessible from everywhere. Google bookmarks do a reasonable job, if you don't have more than say 20 of them. Useful for sites you visit regularly, but not for storing bookmarks 'on the go'. They are just too difficult to maintain, especially if you don't want to install an additional toolbar.
About 2 years ago I started with Digg and was pretty happy. The main purpose was too keep track of what inspired me. So I got a timeline of pages I dugg. After a while I also started to use Digg to search for information and Dugg them to remember. It was pretty useful, but I also noticed that my use reduced and I found other ways to keep track of information.
For a very short time I tried Delicious , but I never liked it. It appeared to be far to complex, I only liked the 'bookmarklet' what made bookmarking a lot easier as in Digg.
StumbleUpon was the next thing, but the toolbar is a too big barrier for me. I appreciate screen real estate and I dont' want to clutter the screen with additional toolbars. I do sometimes use it, but not very frequently.
In the meantime I learned that aggregating information into 1 list became a very useful way of sharing information. Also since the number of RSS feeds that I started to read increased significantly and I like to share inspiring documents. First aggregator I worked with was Plaxo , primarily because I already used the service to keep track and synchronize contacts. However soon there after I started to use FriendFeed and I'm still a very happy user. I like the vibrant users and 'likes'. For any new bookmarking services for me to try, I first look if they are supported by FriendFeed. That is how far it goes now.
In the meantime I also have been playing with Mixx, mainly because I've the feeling that Digg isn't moving forward. Digg really should add a simple to use browser 'bookmarklet'. Recently I also started to use Diigo and I have to say, the 'highlight' feature really adds something extra.
So when do I use which service? Since I aggregate all into FriendFeed it doesn't really matter anymore. I use the service that is most convenient. I always hope to find an obvious Digg, Mixx, Stumble, etc.. button.. and if that is not there I will most likely use Diigo. It's all about ease of use.
Let me know, what do you use?