Malcolm Gladwell (who I've taken a mild swipe at in the past) has an excellent article in the New Yorker called Small change which argues that Twitter isn't the tool to use for a revolution or social change. Having used Twitter and other social media as part of the Alan Turing apology campaign I've had some direct experience of social change through social media.
And I agree with Gladwell.
In the past I've been pretty unhappy about the Twitter revolutions that seem like Western self-congratulation more than actual change. The ridiculous Iranian Twitter revolution was something I criticized at the time and got pilloried for it by enthusiasts. Yet Ahmadinejad appears to still be in power.
I've even criticized people who change their Twitter icon to include a poppy to commemorate the war dead. It seems like doing almost nothing to give the appearance of having done something. Go out and buy a poppy and wear it.
The reality I saw with the Alan Turing campaign is that Twitter is a useful amplifier(*), but it's not the actual cause of change itself. I'm grateful to the people who tweeted about the campaign, but the apology came mostly through strong media reporting (and Twitter amplification of that) and people signing their real names and addresses to a public petition on the Number 10 web site. More than 30,000 people stood up and were counted. They had some 'skin in the game' because they had to identify themselves publicly as wanting change.
A tweet or retweet of a social cause may help spread the news, but it takes just as much effort as retweeting a LOLcat. People shouldn't stop tweeting the things they believe in, but they should do more if they want to support a cause. Donate some time, or money, or write a letter to the government. Above all take some action that takes more effort than clicking Retweet.
(*) Note that amplifiers amplify noise as well as signal. IMHO That's what happened with the Iranian "protests". And, of course, something quite dangerous came out of that: Haystack.