Although people might tell you that nobody cares about this stuff online, unless you’re talking to your closest friends and family, your audience will expect or at least appreciate it if you put some effort into your writing.
Think of it this way: it takes time for someone to read what you write. Why not respect your readers’ time and intelligence by making sure your text is clear and free of obvious errors? You will look smarter, worth taking seriously. It’s your ethos, (your English teacher/professor might have told you about this), the way you come across to your reader, the face you put forward to the virtual world. Better good than bad. Better thoughtful than thoughtless.
If readers know that you respect their time and attention, they’ll be a lot more likely to read your stuff carefully and respond to it. Don’t waste their time, and they’ll be more likely to give it to you.
Adapted from Writing Spaces Web Writing Style Guide, available under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported (CC BY-NC-SA 3.0) License.