If someone has taken the time to reach out to you on social media, it is best practice (and polite) to take the time to respond to them.
The quicker you respond the better, but within 24 hours is a good aim. Remember, if it’s a customer service query, take it off air by asking the customer to call, email or direct message your with their details.
Replying to people on social channels who have reached out to you also demonstrates to others that your account is active, responsive, and worth a follow.
Check through your notifications on your social channels and ensure you have responded to all queries and comments. Even if the comment seems mundane, a simple thank-you will be appreciated.
Other articles related to responding to your customers that you might find useful are: and How to deal with customer service questions, Talk to your customers and How to make your business more 'human'.