Social media builds reach. PR builds the credibility that makes people trust that reach. Brands that only invest in one tend to plateau — loud but not trusted, or trusted but invisible.
The signs to watch for
If competitors are getting quoted in press coverage you should be part of, if your last funding or product news went out on social alone, or if journalists can't find a media kit when they look for one — that's a gap worth closing.
PR and social aren't competing budgets. A press mention gives social something credible to amplify, and an engaged social audience gives journalists a reason to pay attention in the first place.