I haven't really done a lot of UTAU stuff in a while, wow... I guess I'll talk about the main websites I use for vocalsynth audio things in general.
Clypit - Uploading wips of all kinds, to share with people. I deliberately keep this as a dumping ground, and I'm actually kind of disappointed that it's starting to become a little more social. I've started adding to every upload description "I don't accept clyp followers" and yet people keep trying to follow me... I do hope this website stays creator oriented, though.
Soundcloud - Easiest place to upload things without having to put a lot of effort into the visual. I don't care for the community, and I was actually prepared if soundcloud died (had my files backed up and everything). Occasionally I might search for new music here, but mainly I do that on...
YouTube - I spend so much time here. Lately, I really love being able to find new producers and leave positive comments on their work. I think the advantage of being in a niche like vocalsynth is that individual comments have more impact. I've also got Youtube connected to Twitter so it's easy to share things.
DynamiVox is working on a vocalsynth-specific platform, though!