Let's face it. The internet is a smorgasbord of distractions.
If you don't tell your social media visitors exactly what you want them to do, you're giving them a reason to wander off to one of those distractions. That's where the call to action comes into play.
A call to action is a simple line of directions telling people what to do after reading your social media message. For example, if you are spreading word about a charity's service you can include a call to action at the end that says, "Donate now at www.charitysite.com!"
Do you see how that one simple sentence easily tells people what to do, when to do it and how to get it done? Without telling them, they'd have to do some work to figure out how to help, which is asking a lot from people you need donations from.
Call to actions work. Find out for yourself. Create one now at www.facebook.com!