It's a similar fashion to the announcement of the premiere date for Luke Cage, which was slyly announced by lead actor Mike Colter during the Daredevil season 2 premiere back in March. As Variety reports, this makes Luke Cage, the story about a bulletproof black man putting on for the city of Harlem, the third Marvel series to receive a second season order (Daredevil's second season premiered in early 2016, while season 2 of Jessica Jones doesn't have a release date, but might end up dropping in 2018 (considering that Marvel's Netflix line-up already features the first seasons of Iron Fist, The Defenders, and The Punisher).
When we spoke to Luke Cage showrunner Cheo Hodari Coker back in September, we specifically asked him if there were any plans for a second season. "If you've heard anything, please tell me" was his response, which was echoed whenever anyone asked about it, and judging by his tweet about the news, he's just as ecstatic as the multitude of Luke Cage fans around the globe.
There's a lot that Luke Cage left hanging at the end of season 1; Luke was on his way back to jail after the massive Seagate breakout. Mariah Dillard and Shades were left walking the street, down to continue working the angles. Willis Stryker (a.k.a. Diamondback) was left beaten, but not dead, so it's likely he'll be gunning for Luke some time soon. Claire Temple was holding out for her man, but also grabbing a flyer for Colleen Wing's defense class, which leads right into Iron Fist (as Wing is one of Iron Fist's friends).
The fact of the matter is, we could very well see Iron Fist, The Defenders, and Jessica Jones happening before Luke Cage returns for his second season; that's a helluva lot of Marvel to run through, and could change everything for where we'll see Luke in a few years.