This is the absolute nub. No single method of communication carries THAT much weight on its own anymore - we live in a fragmented world and we have to adapt to that. Even compared with two or three years ago, there's less of a chance to use one platform to reach all the people. And crucially, even if it you DO reach all the people, seeing a message once is for the most part not enough to get people to do anything different to what they were going to do anyway.
We need to be nudged a number of times before we act.
Sometimes a library will set up a blog, and try really hard with it, and after 6 months be really disappointed that they're only getting 100 views for each post. But the thing is, it's a few people seeing messages from the Library AND on Twitter AND on a digital screen in the building which tips people over into thinking or acting differently. Click the little stats icon on any one of your library's tweets and be stunned at just how few people actually saw it. But that's okay too, because all of your communications channels combine into a holistic presence. It's about building ambient awareness rather than trying to hit loads of people with a one-off message and expecting that to produce a significant result.
It feels like you're doing a lot of work for not that much reward across a series of channels, but actually the reward comes from how they work together rather than individually. Which is why having a strategic approach to coordinate your messages and to know what each platform is really for, is ultimately worth the time it takes to prepare...
If you want an analogy, think of communication as being like tapas! No single dish is that significant on its own, but taken as a whole it's a really nice meal.