Background Dating allows you to meet people you are really interested in. Each user specifies a set of keyphrases (usually these are first person statements) that best matches his or her personality. They can later edit this set of keyphrases, and any user can search for other users that share their background.
Signing up (as well as using the website) is free.
The server side for the real time chat is written using the Tornado web framework and server. Messages are being transmitted through Redis (as it has a great pub/sub implementation). Data is stored in both PostgreSQL and Redis (for example, the keyphrases are stored in PostgreSQL and number of new messages for each user is stored in Redis).
Here is a more detailed walkthrough about the technical details (in Russian): http://habrahabr.ru/post/159693/