When you consider Dipity as a lifestreaming app, it is kind of cool but not necessarily a standout in this already pretty crowded field. Where Dipity truly stands out is the fact that it can be used to build timelines on any topic that you can think of and many of them have already been created.
Using lifestreaming as a building block to get the rest of the site in motion would seem to me to be a pretty brilliant stragegy. To learn more about what they are thinking, listen to the Dipity Dukes episode of Net@Night.
Plaxo has long had a negative reputation due to their overly-aggressive tactics at acquiring new members in their earlier days. They seem to be working hard to change that.
And they are doing some really innovative things like Open Social Graph. It will be interesting to see where this goes.