Social media communication is rather informal, plus if someone sends your company a direct message then you already know their name. That’s why we’d suggest addressing senders by their first name to shorten the distance. If a profile picture indicates that you are dealing with an older person then you may decide to put “Sir/Madam” at the beginning of your message, but it’s pretty unusual in social media communication.
Remember to use emojis, which are the second language of social media.