I think it really depends on the person and the aspect of writing that you are questioning. For instance, grammar and spelling have certainly taken a hit from technology. I, myself, have noticed the change in my spelling from a constant dependence on technology like spell check. Teachers are also seeing a lot of grammatical and spelling errors from constant texting and other quick communication. We see a change in the style of writing as well. Social technology often leads young people to a lot of practice with informal writing. Many students have no concept of formal writing at all.
On the other hand, technology has forced many business personnel to become better writers. They have to write e-mails, reports, and other such items much faster than they did in the past. Personnel must become far more proficient at writing in order to communicate through fast paced technology.
From a deeper perspective, technology can both aide and prevent good writing. Some read more and learn more with use of technology. They might gain a higher level of vocabulary and learn much about the world around them. This would tend to make them better writers and communicators. Others might read less and waste their time in frivolous pursuits with technology. These people would tend to be worse writers. They lack the skills that are expected of their peer group.
I agree with those who say it can work both ways. To the extent that we no longer read as many books as some people did in the past, technology can definitely lead to worse writing, because much good writing is the result of wide reading. However, to the extent that technology allows us to have access to an enormous amount of ready information that can help us improve our writing skills, it can be beneficial. Here's an example of the sort of information I have in mind (take a look at that index!):
I think that people get into the habit of abbreviating with text messages and sometimes even in e-mails. Also, Microsoft Word's spell and grammar check functions can make people lazy about proofreading. On another note, my iPad changes things after I write them, and I don't always catch it because I type so fast. It loves apostrophes. It puts them everywhere!
Technology itself is neutral. It can help you be a better writer by allowing you to easily do drafts of your work and then allowing you to pick and choose between the drafts or parts of the drafts. On the other hand, it can help you fall into bad habits like those that you might use while texting.
But the technology itself is neutral. You are the one who chooses how to use it.
I think this can go both ways. In many cases, it results in writing without thinking which can result in grammatical errors as well as incoherent thoughts. It also has resulted in loss of spelling skills since most programs will automatically spell check what you write. While it can be much easier to research a paper or topic, it's also easier to just do a "brain dump" rather than organize your thoughts and ideas before assembling them into a paper.
I think the question probably references the effects of email, Twitter, and texting on the quality of writing. It ultimately boils down to the individual. You have to differentiate between audiences when you write, and understand, as many of my students fail to do, that what is appropriate in a text is not appropriate in a paper. I will say this for technology: It has certainly made research easier.
This is not a question that can be answered in a definitive way. It all depends on the person in question. Let me give you some perspectives.
First, if technology takes away time from important things like reading and writing, then it can make people worse writers, because they dedicate less time to it. For example, some people are too into video games. For this reason, they may never write. Others use technology to surf the Internet endlessly and in the process read things that are not very good. This waste of time will make a writer worse.
Second, there are those that use technology in a wiser way. They use it to get information in a quick way and the like. I would say that for these people, technology makes them into better writers.
In the end, it all depends on the person in view.