[ac4tw]

I don't think there are any 3 that are exactly what you ask about ("chat-on-blockchain thing").

However, from my understanding of Telegram's ICO and "leaked" roadmap, they appear to be doing a decentralized chat the may leverage their own blockchain for some features (identity, tokens etc.).

I think Status.im is another example, but it's currently early access I believe.

My own product, stealthy.im could be another example. The web product is in production. It builds upon blockstack, storing identity and storage pointers in the blockchain.

So that's 3. I could probably list a few more, but again they wouldn't strictly satisfy your precise criteria in one or major ways. I think @jerf's response is really good--for us the "why" of blockchain is in identity and providing a verifiable decentralized storage network through blockstack. The underlying bitcoin blockchain wouldn't be suitable for message storage or sdp handshaking due to latency as another person pointed out.