Writing content for social media.
1: Your tone of voice shouldn't be different just because you're using social media.
2: See rule 1.
Don't act cool.
There's no need to make your brand's tone sound a bit more 'cool', just because you're using social media. If you tried that in real life, people would think you were either mad, a ventriloquist, or possessed.
Don't tell porkies.
Your brand can use the same tone of voice in social media as it would anywhere else. Just be honest about what your business does, and you'll avoid coming out with such banality as "We just released a new product. LOL!!!".
Have a chat.
By the very nature of the word 'social', you're starting a conversation. Everything needs to sound like it's being said, rather than dictated. Be human - there isn't a brand on Earth that shouldn't be.
If you're stuck, ask a copywriter.
Showing your brand's writing to the world can be daunting. So don't suffer in silence - get help. I provide language consultancy services to help brands use their voices. You can even ask me to write style guides, so you can check what you've written. Or I could write for you, you know, being a copywriter and all...