Friendfeed aggregates and stores all the activity that is fed into the system. Most FF users bring in their Twitter feed, in effect storing all their tweets. It works a little bit like Google Reader, once it’s there, it will always be there, even if the original is deleted.
The advanced search features of Friendfeed makes it a pretty good twitter search alternative. It even supports real-time, so you can make your own twitter news monitors.
I wrote this for the Online Journalism Blog, read the rest there.
I have been posting some ideas and thoughts about Twitter and friendfeed on both of their services. Mostly questioning how 

Kasper Sorensen is an online multimedia journalist with everything that encompasses. Hook up with him on