While there are certainly more email users than any web service out there, Gravatar isn’t quite as buzzworthy as Twitter; it’s a more specific service after all. However, because of this Twitter Images service, extracting an avatar is much easier than Gravatar’s implementation and could gain more traction as a legitimate avatar solution on blogs. I won’t be surprised if Twitter scooped up this little project for itself.
On the other hand, being dependent on Twitter—whose popularity still causes downtimes to this day—may not be such a good idea for critical endeavors, and it may be more advisable to go for the service whose sole business is avatars (or if possible, identity management).
Gravatar and Twitter don’t have to be adversaries. I’d want Gravatar to take the high road and embrace all the popular identity channels, be it Twitter, Facebook, Google, Yahoo, MobileMe, OpenID, etc. Or should one leave the multiple avatar sources feature to the developers just like you can have different login and identity options on blogs and web services? Perhaps Mix Online’s Incarnate is the right way to skin the cat.