Slacktivism is a failure if you consider it to be a form of activism, but as a form of value signalling, it is terribly effective. We underestimate the value of signalling at our peril.
Some parts of our moral landscape appear to be biological in origin: a revulsion reaction to the idea of incest, for instance, appears to be the result of the sexual imprint process (and we can tell because siblings separated at birth actually have a much higher likelihood of ending up together, while unrelated children who spend a lot of time living together prior to puberty, even in non-family settings like boarding schools, have unnaturally low rates of sleeping together as adults). Others, however, seem to be primarily controlled by culture, specifically by cultural manipulation of empathy and shame. (We dehumanize enemies in war via propaganda, in order to eliminate the empathy we might normally have toward them & the shame we might hold for killing them, and instead substitute a new set of rules around what kinds of killings are shameful. If the domain over which empathy operated was biologically determined, the flexibility that makes modern warfare possible would not exist.)
Some people lack empathy (or its effect on them is abnormally low), just as some people lack the in-built biological drive to avoid incest. Shame works to police such people. Ultimately, shame serves to punish people who engage in violations of a culture’s idea of moral behavior by lowering social status (and with it, access to various resources — particularly, other people). While we should still be concerned about sociopaths (who have a strange sort of superpower: their actions are not constrained by the force of empathy, and they lack the self-control to respond reliably to shame), the worst excesses of garden variety empathy-deficient narcissists can be avoided by judicious application of social pressures.
Value and virtue signalling is a major way in which a culture indicates what behaviors are considered acceptable and what behaviors are not considered acceptable. The other major way in which acceptable behaviors are signalled is punishment; however, punishment requires that the behavior be practiced and the culprit be caught. Value signalling might include describing counterfactuals or hypotheticals about punishment for breach of acceptable behavior (ranging from fantastical visions of hell to fairly concrete legal sentencing guides).
Slacktivism is a form of weak value signalling, wherein large numbers of people expend small amounts of effort in a token representation of the ideal behavior. While strong / costly signalling would send a more powerful message, it’s not accessible to most people, and so the number of people engaging in it will necessarily always be small. Weak value signalling, at scale, is actually more potent: after all, costly signalling is more desirable both to those with sufficient resources that the marginal cost of signalling is smaller than its apparent cost to its target audience (philanthropists) and those with nothing to lose for whom costly signalling also represents an out (terrorists), neither of whom can be trusted to be an accurate representation of group norms. Mass action, on the other hand, is group norms embodied, and low social cost makes scale possible. As a result, slacktivism allows small changes in moral values to propogate quickly from the majority who already accept them to the minority who haven’t yet, in much the same way as the Game of Life: if you’re surrounded locally by people who send a particular signal, you’re more likely to send that signal, until the signal reaches some saturation point.
Slacktivism, by changing the collective value system, also encourages acts of costly signalling in the same direction as that weak signal. Popular causes get donations (although the ratio of weak signals to strong signals will always be large).
Criticisms of slacktivism tend to hinge on the idea that it’s a substitute for activism — that everyone who changes their profile picture to a flag might otherwise be staging a sit-in or assassinating a congressman or otherwise helping make real changes. But, slacktivism is better modeled as a form of collective social control of activism: a means by which various causes have their perceived importance ranked.