I know it's fashionable to be dismissive of things.

No one wants to hear how fantastic your yoghurt is, even if it's the microbial equivalent of a kitten dipped in crystals singing Puccini. We want to hear how buses are slow, politicians are fascist pigs or the actress looked like she was permanently recovering from general anaesthetic.

After the Boko Haram kidnapping, and #bringbackourgirls, we heard a lot of sniffing about why social media activism was ineffective. Just a conscience-soother for a few Westerners who want to enjoy their lattes without pesky guilt trips.

I didn't agree with that idea then, and I certainly don't now. If the last fortnight has shown me anything it's that social media campaigning can be incredibly effective.

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Especially when the campaign goals are social change.

I'm talking about the hashtag activism of #YesAllWomen and #AllMenCan. These were the campaigns that sprang up in the aftermath of the Elliot Rodger massacre of six people at the University of California in Santa Barbara last week, when the world fervently debated whether Rodger was sick, poisoned by casual Western sexism or both.

#YesAllWomen was a backlash against media dismissal of sexism, which aimed to show the world the everyday sexism women fear and experience. #AllMenCan, where men show support for feminism, sprang up in support of #YesAllWomen after it got a fearsome online backlash.

Basically a lot of hashing, lashing and syntax smashing. But the campaigns were beautiful examples of exactly why hashtag campaigns can achieve their goals.

How do you achieve change? You start by raising awareness of the problem, demanding a change, and helping victims feel empowered. #YesAllWomen is a stellar example of how to do this.

First, it raised awareness of the problem. The campaign itself had tweets like "girls grow up knowing it's safer to give a fake phone number than turn a guy down", and "'I have a boyfriend' is the easiest way to get a man to leave you alone".

Because he respects another man more than you.

They bring home that this isn't just an overseas epidemic. I've been giving fake numbers or mentioning fake BFs since I was in Year 10. This is as relevant in New Zealand as it is in New Jersey.

New Zealand has a horribly complacent attitude to continuing the discussion on sexism. Growing up, if I ever protested that something was sexist, I was met with "we love women, we gave women the vote, don't fuss and have a biscuit".

But Twitter is built on first-hand everyday accounts so we saw the multitude of experiences of everyday sexism . When you have such overwhelming real-life accounts, it becomes much harder to rely on our abstract notions of our own great attitude to women. It forces us to think about ourselves.

Hashtag activism can also convey the power of desire for change. #YesAllWomen, according to writer and political activist Zerlina Maxwell, reached out to mainstream society in a way that usual feminist campaigns don't. It triggered #AllMenCan where swathes of ordinary guys expressed disgust with ongoing sexism.

The campaigns have drawn on various, distinct groups for support. Twitter has been able to show how broad the support base is. This shows that desire for change is widespread and mainstream. So it's much, much harder to ignore.

Hashtag activism can change the individual's attitude to sexism, just from them looking at the experiences and support. It can make you think how you can change, even if it's just not laughing at a rape joke. As social change rests on change at an individual level, through accessing the individual, Twitter makes itself invaluable as a campaign tool.

Last, hashtag activism directly helps victims. Victims can feel like they're not alone, it's not their fault and that people really do care. If only one victim feels better, it's a success.

Yes, yes, I hear you sigh, it's fan-daby-dosy that it can achieve social change. But can hashtag activism get actual measures introduced?

Well, let's put my arts degree to eventual use. In 1984, the UK No 1 hit Free Nelson Mandela by the Specials would have been dismissed by shoulder-pad wearing, fizzy water-swilling cynics. They would wave a dismissive fag at such conscience-soothing "pop culture activism".

But that song was instrumental in galvanising international support for Mandela, support which made him a global hero, pressured foreign governments into enforcing destructive economic sanctions against South Africa, and forced them to consider freeing Mandela and stopping apartheid.

Not just a song people stuck their digital watch-clad hands in the air to.