The Women's Equality Party are all over Facebook and Twitter gathering momentum for new members joining.
Facebook page.
Website
I'm not a member and this isn't an advert - how big do you think this is going to get? Theoretically as a party whose natural constituency is 50% of the population they could become huge. Or not. The Green Party started out single-issue and is becoming a lot more widely accepted and supported as it matures. For that matter, Labour started out as championing the rights of the working classes and has changed into a "mainstream" party over the decades. Is #JoinWE going anywhere?