At this particular moment, people are looking/searching for the next big thing so if you come up with a great idea and platform then there's no doubt it can be a success.
So, do you think it is ever worth opening a new social network, do you think they will ever take off?
Yes.
They have to compete with sites such as Facebook, Twitter, Google+ and others.
All your inputs are appreciated.
Don't focus on getting 1 billion users when you're just getting started. Focus on getting that 1 person who'll bring traffic to your platform. Pay them if you have to maan (it's called advertising). They could be a popular blogger, youtuber, online or offline tv/radio presenter, forum admin, facebooker or well known in whatever field or platform.
Also make sure these people post content and recommend it on other online/offline portals. You see, for something like a social network you don't have to focus on how "You" can get 1 billion users. You have to focus on how other people are going to help you get to that 1 billion users.
It's all about getting those popular individuals who'll post valuable content that is engaging, informative, entertaining, educational, political etc. to keep users coming back as well as recommending the platform to others.
And how you're going to present your platform is the most crucial thing, you have to remember that when all these platforms started, they were not designed for everyone or the general public. Each one was targeting a specific group of people or niche. The users of these platforms are the ones who spread the word especially when other popular brands/people/businesses/icons start using the platform, talk about it and recommend it.
So yes a new social network in 2015 can be a success, and people are tired of using platforms like facebook and twitter they want something fresh (me included).