I really like this article about descriptive norms, and how people are influenced heavily by what they see others doing or hear they have done.
"When people try to change behavior, they often focus on prescriptive norms, telling people what they should do. We often underestimate just how strongly we respond to what other people actually do.
In a classic study, Cialdini and colleagues manipulated the signs that were displayed in Arizona’s Petrified Forest National Park, a site often plagued by tourists who end up grabbing some of the petrified wood to take home as a souvenir. In situations like this, the first inclination of well-meaning environmentalists might be to set a strong prescriptive norm — perhaps by saying something like, “Many past visitors have removed the petrified wood from the park, changing the state of the Petrified Forest. This is bad, don’t do this.” The idea here would be to invoke a sense of shame and severity before asking visitors to refrain from taking the wood. But read that prescriptive message once again. Is there anything descriptive in there? Yes, of course there is. That message is not just telling you that you shouldn’t take the wood — it’s also telling you that most other people do. In fact, people were actually more likely to steal wood from the forest when they saw the sign telling them how many people tend to do it themselves, even though the very next sentence was asking them to refrain. But when the researchers simply tweaked the message to read that “the vast majority of past visitors have left the petrified wood in the park, helping to preserve the natural state of the Petrified Forest,” the thievery plummeted."
I've seen it play out in various settings myself, things like when you have a pristine toilet cubicle door, it stays that way for quite a while - but once one person scrawls on it, it quickly becomes covered. If someone leaves rubbish beside the bin, more rubbish quickly appears and so on.
So, yes, it's good to think that perhaps 'slacktivism' on FB through profile picture changes etc might actually go some way to changing social perceptions.
I think this differs from the sappy FB memes I have previously complained about (possibly because this is something I agree with!* but also) because it requires a shift in thinking. After all the anti-cancer memes are something easily paid lip-service to, while supporting gay marriage is more controversial, sadly.