In a post to The Brain Fitness Authority blog, Dr. Bill Klemm (author of "Thank You, Brain, For All You Remember. What You Forgot Was My Fault") suggests that social media is not so great for memory, or for grades in school:

Our brain works hard to fool us into thinking it can do more than one thing at a time. It can't. Recent MRI studies at Vanderbilt prove that the brain is not built for good multi-tasking. When trying to do two things at once, the brain temporarily shuts down one task while trying to do the other. ... So what has this got to do with memory? Well,if you try to memorize the first task and the brain immediately switches to the second task, performance of the second task interferes with consolidation of the memory of the first task. ... A study of 517 California high-school students found that grades were lower in those who socially interacted via MySpace, instant messaging (IM) accounts, or who used cell phones.

And I thought my increasingly fuzzy memory was just one of those "getting older" things...