Texting and social networking sites don’t require careful reading or editing. Those are skills expected of literate individuals.
They also don’t require writing in complete sentences. Twitter, for example, only allows 140 characters. Consequently, many students write in fragments and run-ons.
This also has an effect on word choice. Typically, students use slang and abbreviations. Consequently, many students start using slang in formal writing because it’s part of their everyday writing and speech.
There also is a negative effect on punctuation, especially when texting. It ignores language and writing conventions. Many students have no idea when to use capital letters. They don’t capitalize ‘I.’ They don’t capitalize proper names. You’re/your, there/their/they’re, and its/it’s become interchangeable. Punctuation is haphazard.
Also, texting and social networking sites don’t require practicing handwriting; therefore, many students can’t write legibly. Every year, I am amazed by how few of my eighth-graders are able to read or write cursive. They struggle with their own signature.