Another thing to think about here: if you save lives by donating your organs, the organs will probably go to elderly people who are not signed up for cryonics, and will probably die in the next few decades regardless. So you will have saved a few decades of infirmity. On the other hand, if you are revived from cryopreservation, you will probably be revived to immortal, healthy life. So, if the Singularity/some other form of immortality does not happen until more than a few decades after your death, you can save more years, with greater average quality of life, by cryopreserving yourself.

ETA: Also, if the people your organs go to are signed up for cryonics, then getting your organs still wouldn't make that much difference to the total number of years they live.