The main difficulty in writing an essay on this topic is that the general question is far to broad to be covered in a single book, much less a single essay. The major task of your introduction will be to narrow the scope of your essay to something manageable within your overall word count.
One way to do this is to refine your concept of a “great” invention and think about “great for whom?” or “great for what?”. For example, the internet makes it easier to distribute visual imagery to a wide community. One the one hand, much of the Arab spring has been facilitated by circulation of pictures of street protests (and of torture and oppression). On the other hand, the internet makes it easier for paedophiles to distribute child pornography. While many more people can see images of great painting online than can visit museums, is a .jpeg of the Mona Lisa, with its limited number of pixels, the same experience as looking at Leonardo’s original? If museums have limited budgets, and the money going to support digitalization and wide distribution of simulacra, as it were, is taken away from money which preserves, displays, and protects original art, can the internet be seen to have benefitted artists and audiences?
In your introduction, choose one narrow topic concerning the internet as an invention and carefully examine both ways in which it can be called “great” and ways in which it can be described as somewhat continuous with other technologies, and then think about the uses to which that aspect of the internet has been put and whether they can be considered beneficial to humanity.
You might look at the work of the Toronto School of communication (a group of communication theorists centred at the University of Toronto in the 1970s and 1980s) as a theoretical starting point.