I think that this is a really reflective writing assignment. I don't think it's a bad idea to speak with your teacher privately to gauge their thoughts on the topic because from what I can see, it's really profound in terms of its reflective quality. In what has been offered here, I think that the writing assignment is talking about how your use of social networks has impacted the content and style of your writing, in general. This is going to require you to reflect on two fronts. The first would be how your writing has changed over a particular period of time. For example in the last three years, what do you see different in your writing? Are you writing in a more profound manner? How has your sentence construction in your writing changed? Has it changed? What were you writing about then and what you are more occupied with writing or thinking about now? Are there more emotions in your writing? Is there more complexity, less of certainty? These are all questions that you have to address. Then, the next level of reflection is how your use of social media and composition of thoughts on it has helped to convey this change. For example, when you were using social media three years ago, what were you using it for then and what are you using it for now? Back then, were you using social media as more of a "Hey, What's up?" thing as opposed to now where you are posting comments about your feelings on facebook walls? I think you have to go back and look at what you were saying through social media back then and now. Interestingly enough, I think you have to examine what social media forms you were using then and what platforms you use now. Are you using Tumblr? Are you using Twitter? Discuss some of these platforms, what they enable you to do and how your needs in using them might have changed. Remember a good working definition of a memoir in all of this given by Gore Vidal:
a memoir is how one remembers one's own life, while an autobiography is history, requiring research, dates, facts double-checked.
This means that you are driving the narrative in assessing who you are in your writing in social media. It is wide open and this might be where it is kind of scary. Yet, if you really reflect on this, I think that you will not only have a solid memoir in hand, but might gain some insight about yourself. All things considered, that's not a bad thing for a school assignment.