What is the timeline for Lightning Network's availability? I have been reading a lot about this Lightning Network and what it does.
But what I still don't know is if this Lightning Network is already being used for Bitcoin or when Bitcoin will start to use it ?
Roasbeef just announced the first Lightning transaction on Bitcoin mainnet today: 
https://twitter.com/lightning/status/938453165967515648

It was using a multihop payment via all three major implementations (lnd, eclaire, c-lightning).

The Lightning Integration tests are now at 75/75 working: 

The three teams are now working on a beta release each. I'd expect this to be released in the next months.