The writing prompt reminds me of one of the TOEFL independent writing tasks. As a student, who did pass the exam and got 28/30 points on writing, I believe that I may help you a little bit. However, please take my answer with a grain of salt as I am just a student who learned some core ideas behind effective written communication.  
  
Your essay is quite strong and shows a clear paragraph structure. Nevertheless, your introduction lacks a clear thesis statement. Now, I can advise you to spend a few minutes before writing the essay on preparing a short outline. Nothing fancy, it should just state whether you agree with the main idea or not. Below it, you should write two to three reasons why you agree/disagree. Three is better, but if you do not have time or can not come up with the third, it is always fine to have only two. For instance, in your essay, you have two reasons:  
  
1) Spoken communication is more powerful because nowadays people can easily get in touch with others through the internet.  
2) I find that oral (=spoken) communication is more powerful since it requires direct interaction between speakers.  
  
When having this little outline you can actually start writing an essay. And it will be easier, too, because you already know what to include in your thesis statement.  
  
About the thesis. It should clearly present your opinion and be arguable (=contain your reasons). For this task, your thesis could have been something like this:  
  
"I totally agree that spoken communication is more powerful than written because nowadays people can easily get in touch with others through the internet and because it requires direct interaction between speakers."  
  
See, now the reader will know what exactly the essay will be about. If you are more into advanced grammar, you may also use colons. For instance,  
  
"In my opinion, spoken communication is more powerful than written: it is easily accessible through the internet; it requires direct interaction between speakers."  
  
Again, I am not a native speaker and may have made a mistake since it has been several months since I took the test. Nevertheless, you get an idea: lists can be utilized in the thesis to, well, list your reasons.  
  
Now, you may also want to meat-up your introduction. To do so, you may start with some general statements. E.g., people have been communicating for decades and throughout have invented different kinds of communication. After such a statement, you may write a sentence about opinions that people have about these types of communication. For instance, your sentence here: "People, who disagree with the idea that spoken communication is more beneficial than written communication, may argue that written communication is more accessible, as messages can be viewed anywhere." is perfect. However, you may want to write about the other side of the conflict as well. For example,  
  
Some people disagree with the idea that spoken communication is more beneficial than written communication since they believe that it is more accessible, while other people may find spoken communication more enjoyable as they can see other people's reactions.  
  
So, to sum up, the introduction would look something like this:  
  
People have been communicating for decades and throughout have invented different kinds of communication. Some people disagree with the idea that spoken communication is more beneficial than written communication since they believe that it is more accessible, while other people may find spoken communication more enjoyable as they can see other people's reactions. In my opinion, spoken communication is more powerful than written: it is easily accessible through the internet; it requires direct interaction between speakers.  
  
It is a draft, for sure, but may give you an idea. Now, having a clear thesis statement, you can easily start each paragraph with a topic sentence (since you have written them all in the outline, remember?)  
  
Overall, your examples are sound and understandable, so just focus on a thesis and topic sentences.  
  
Keep up the work and good luck!