Wiktionary shows the pronunciation of moiety as /ˈmɔɪ.ə.ti/, which I think agrees with the audio versions at merriam-webster.com and howjsay.com. (Be warned that both those links produce audio when clicked.) Anyhow, from an 1832-vintage quotation for sense 1 at wiktionary (meaning half), I have the impression that at some times or places the word has been pronounced with 4 syllables instead of 3, and rhyming with society:
From New Holland the emu,
With his better moiety,
Has paid a visit to the Zoological Society.
Has its pronunciation changed since then? Or was this perhaps meant to be merely a half rhyme?
Edit: A half rhyme, or near rhyme, or imperfect rhyme, entails consonance on the final consonants of the words involved. Moiety (/ˈmɔɪ.ə.ti/) and society (/s@"saI.@.ti/) rise a little above that level, but not to the level of perfect rhyme, which according to wikipedia's rhyme article, entails having final stressed vowel and all following sounds identical. But /ɔɪ/ and /aI/ are not identical.