Merely having a social presence doesn't cut it. If you're not paying attention and communicating with your followers promptly, it could be doing more harm than good. There's a pretty decent chance that they're following you specifically to communicate with you.
Sprout Social surveyed 1,000 social media users and found that:
25% of the people who follow you do so because they want to communicate with you
15% unfollow a brand simply because they don't get a reply
So if you're going to be on Facebook or Twitter, be active and be yourself. Don't try to be funny if you're not and it doesn't fit your brand. And don't bombard them with constant promotions. Finding the perfect balance will probably take work and some trial and error. Still, considering that 75% have gone on to buy something because they saw it on social media, it's worth it.