"Helping"? Yes. "Causing"? Not fully.
There are many reasons why literacy in America is in the state that it is. Computers play their part in that Instant Messaging (and Text Messaging) promote using abbreviated and "cutesy" words instead of writing in complete sentences. Even though most people (like myself) type faster than they can write, many still feel the need to make things as succinct as possible.
Another problem is, for the most part, kids see reading as a "chore" (and I'll give the "Harry Potter" series a lot of credit for getting kids of all ages to pick up a freaking book without being forced to). Television and video games are a more tantalizing choice for entertainment for kids, since it's a matter of instant gratification.
Also, even schools don't promote literacy as much as they promote increasing their test scores. They don't teach children English, they teach them what's going to be on the SAT's so that the overall score of their school will be better. While learning "high:low::dark:light" is great and all, it's certainly no substitute for learning how to read and compose a paragraph.
If you want your kids to be more literate, here's a question: How many parents take their kids to the library? That's one place to start.
