Duolingo gets success because it offers all elements for an efficient learning:
In the middle of the afternoon, you are at work and you start getting bored: 10 minutes on DuoLingo to improve or learn a new language.
After dinner, skip your favorite TV show and watch another kind of video: 20 minutes on an opencourseware platefom like the Microsoft Virtual Academy or the MIT OpenCourseWare and take some notes (10 minutes). For more interaction with other learners and to get a certification, you can attend to a MOOC on Open2Study, edX, Coursera or NovoEd. And if you do not find an interesting topic on these platforms, check the MOOC portal designed by the European Commission.