Very very very few new technologies have 'killed' an old technology. What new technologies usually do is do part of what one or more old technologies did much better while making old technologies focus on their strengths.
- Print Press vs Books vs Radio vs TV vs Internet vs Mobiles
- Horse and Cart (now used just on some farms) vs Canals (now used for living and pleasure trips) vs Trains (still king for short-medium distances and large loads across land) vs Trucks (smaller loads across loads with end-to-end delivery vs Boats (long distance, slow spoiling loads) vs Planes (passengers and high-value fast spoiling loads)
- Letters vs. Phone Calls vs. Emails vs. IM/Micro blogging
- Film Camera (still used for high-end professional photos) vs Digital Cameras
One of the few times I can think of when one technology replaces another is when it is a direct replacement: data tapes vs floppy disks vs CD-RW vs DVD-RW vs USB/Flash/SD Cards - but even this still has examples of each 'generation' still being used
...and from using Google Wave, there is no way it will replace email any time soon if ever! It is a great collaboration tool, but isn't an email killer. It will likely replace email's mis-use as a collaboration tool which is what it was intended to do.
For anything to replace email, it would need to:
- Non Proprietary which almost all social networks and media is not
- Be a free both to users and in terms of the licensed technology
- Be an open standard which almost all social networks and media is not
- Do all things much better than email. Most alternatives do only a few things better than email.
- Be as easy to use as email. Hard to get easier than 'compose', 'reply', 'forward', 'delete'
- Not dependent on any one provider
- All data is portable
Email is so flexible it allows LOTS of bad practices - but it also allows lots of good practices. Most social network and IM system prevent most practices but also don't allow many good practices. (e.g. think facebook messages from groups: limited recipient size, no html, no tracking, etc)
So email dead? No way! Facebook and Twitter dead: much more likely :-)