The final episode of Breaking Bad had 10.3 million TV viewers in the US

"AMC’s Breaking Bad capped its historic run last night, delivering a series-record 10.3 million viewers, including 6.7 million adults 18-49. Ratings for the final episode were up 300% over last year’s finale, demonstrating the remarkable growth of this iconic series. An expanded one-hour Talking Bad after show delivered 4.4 million viewers, including 2.9 million adults 18-49.
“Breaking Bad is simply unique,” said Charlie Collier, AMC president. “It all starts with Vince Gilligan who really only ever asked for one thing – the opportunity to end the show on his own terms. That is exactly what Vince did last night and, as always, brilliantly so. Congratulations to Vince and to every single person involved in this remarkable journey. We’re proud that AMC will forever be known as the birthplace and home of this iconic show and, at the same time, we tip our Heisenberg hat to the fans who made this a truly shared experience.”
Key Nielsen Highlights for the Breaking Bad series finale
9 pm airing – 6.5 HH rating with 10.3 million viewers
9 pm airing – 6.7 million Adults 18-49
9 pm airing – 5.6 million Adults 25-54
Key Nielsen Highlights for the Talking Bad finale
10:15 pm airing – 2.9 HH rating with 4.4 million viewers
10:15 pm airing – 2.9 million Adults 18-49
10:15 pm airing – 2.4 million Adults 25-54
Source:  Nielsen Media Research fast nationals, L+SD 9/29/13.
The Breaking Bad series finale set new records for the show on Twitter, with 1.24 million Tweets from 601,370 unique users during the live broadcast window of both the EST and PST telecasts. The show hit a peak of 22,373 Tweets-per-minute just as the final episode began in the East, and a Bryan Cranston Tweet, “Well, this is it… Thank you for sharing this ride with me…” has generated more than 51,000 Retweets and nearly 34,000 Favorites so far. Over the course of the night, Worldwide and United States trending topics on Twitter related to the show included #GoodbyeBreakingBad, #AMC, #BreakingBad, #BreakingBadFinale, #Heisenberg, #HelloGretchen and #GoodbyeLydia."
Source:  Blog post from AMC, 30th September 2013
Note - This clearly only includes people watching it on TV, not online, or through illegal means
Note - Torrent data here - >500,000 downloads in 12 hours