This can be very hurtful and it feels like a personal attack first step to dealing with this is BREATHE.
It's not the end of the world, you are never going to please everyone. 

Remember that most of the population are very aware there are 2 sides to every story and furthermore there are extenuating circumstances sometimes which are out of everyones control. 

Once you've had 5 minutes to get over it, take a few deep breaths, now we can address the problem. 

Firstly don't ignore it. If it is totally unreasonable or in fact a personal attack on you, you can contact the platform it is on, for example Google, Facebook, Instagram and asked to have it removed. If this is a malicious attack it could also be advisable to block this person off your social media channels. 

If it is a reasonable complaint about not meeting expectations, addressing the issue is advisable. If we take a step back and you are aware that something has missed the mark it is always best practice to sort it out there and then, so it doesn't get to social media, unfortunately sometimes you don't get that opportunity. As tempting as it is to ask this person to delete their comment or deleting it yourself it quite often leads to people thinking you have something to hide, so try and avoid that if you can. 

Responding with the example text below could possibly defuse the situation: 

Dear ______________
Thank you for taking the time to bring this to our attention. We are sorry that you feel let down by our service/product. We would like the opportunity to correct this, if you could please contact us on XXXXX@XXXXXX.com.au and we can work to rectifying this. 

1. Listen to the complaint. Thank the customer for bringing the matter to your attention. 
2. Record details of the complaint.
3. Get all the facts.
4. Discuss options for fixing the problem. 
5. Act quickly.
6. Keep your promises. 
7. Follow up.