I did wonder if Google Wave was ever going to be useful. Apart from a go when it first came out and then a little flourish just a while ago, I hardly used it. Tried. Tried to stir some interest (as much in myself as well as in others) but Gmail was not replaced.
If Wave had happened at the beginning, then perhaps it might have been different. But there was no social media concept then.
Google state that it will not continue to develop Wave but will maintain the site "at least through the end of the year" and will extend the use of the technology to other Google products. This may be interesting since the instant and continuous editing in Wave is amazing.