We have a few messenger services running on the steem blockchain. Although, none of these services are very popular but we still have them. Here is one by @kingswisdom:

https://beta-steem-messenger.herokuapp.com/

Instructions for using the messenger:

1. Open https://beta-steem-messenger.herokuapp.com/

2. Click this sign at the bottom right-hand side of the page 

3. Click 'START' 

4. The messenger would ask for your private memo key. 

You can get you private memo key from you Steemit wallet.

5. After entering your 'username' and the 'private memo key' you can press 'LOG IN' and start using the Steem Messenger.