What if men weren't allowed on Facebook?
slate.com
slate.com
1. Social is insanely crowded, it is such a crowded space and most of the decent niches have been carved out already. Other networks provide nearly perfect substitutes and are already well established an populated.
2. You eliminated 50% of your potential market before launch.
3. Is having 2X chromosomes something that is going to bring people together.
4. This can only work as either an anonymous network (because people will likely just want great opinions from other women who they ALREADY DO NOT KNOW IRL) and that means men and trolls will flood it
OR
5. It has to be a Facebook like social network that really tries to get an actual identity, in which case it is competing with Facebook, which is tough.
I don't see this becoming a huge thing. Would be surprised if they get a million users even in like a few years.
In terms of niches, the 2X chromosomes niche is much bigger than the programmer niche, but stack overflow is still successful.
In reality most sites that are ostensibly for "everyone" like reddit or tumblr end up being dominated by certain demographics and thus start to repel others. If they are smart they will make sure the demographic of women that they most appeal to are those with high disposable incomes or influence who are of the most value to advertisers.
There is Korean website I've known about for the past 10+ years - MissyUSA
It is a Alexa ~ 35,000 global site:
http://www.alexa.com/siteinfo/missyusa.com
So it is not all that large. They have had a women only policy and focus on Koreans in the US - it is pretty active and is has a political impact:
http://www.koreatimes.co.kr/www/news/opinon/2013/05/390_1357...
ii) depressing that a women-only social network is needed. We've had online communities for over forty years so it's kind of disturbing that they all suck so much.
iii) trolls are trolls and it's not just men making threats of sexual violence. Women do it too. http://m.bbc.co.uk/news/uk-25886026 (the tweets were so extreme that the BBC has to filter them, so it's hard to get the tone of the tweets.). Still, this should stop "boobs or gtfo".
In Australia it's against the law, but the courts can make exceptions if you convince them that what you're doing serves the public interest. Things like gay bars and women's spaces do so routinely.
With that in mind, you have a logical hole in your statement "If you only let women on a website, it is going to be about womens issues, etc". Women.com will be about women's issues because it is currently being marketed to the general public as a place for women to discuss women's issues. It will not be about women's issues solely because women are allowed on the site. Women who aren't interested in what other women think about women's issues won't find this site very interesting. In general, women will discuss what they want to, where they want.