A little more functionality might improve the idea, but I dunno if it could be done in a weekend:

Add an OpenID provider and allow users to attach more URLs/feeds to the unified streams.

IDEA:

See your social network

CONCEPT:

Most social sites and many embed microformats in pages. Allow a user to input a URL and automatically gather other pages for that owner of the URL from around the internet. Grab and blend all the feeds from those sites into a single unified feed a la http://soup.io . Additionally gather URLs of all contacts/friends (identified through microformats) and create unified feeds for them. Possibly highlight services you are both on but aren't currently linked, etc. Benefits to user: A consolidated view of their activity (and their contacts) online.

MONEY:

??? Advertising? Auxiliary services?

WHY IT WORKS:

In case you haven't noticed, social is the new black. (Actually that might be a reason it would fail.) Something also to keep in mind is that this approach is much less evil (and internetish) than Facebook/OpenSocial. As for technical feasibility, there are lots of libraries out there that support small pieces of this idea... Just have to put them together in the right order.

Sorry for my dense description, ask for any clarifications please. If anyone has suggestions for a business model, shoot!

A lot of competition. However, something that I have always found that is greatly needed is internet-wide busy search. Some software let's you do it internally but I don't think I have ever seen anyone offer to find a meeting time (and send an appt in ics) that works with all published web calendars.

You got it exactly.