The Multi-tasking Scatterbrain

Nobody should be surprised that people who multi-task a lot are easily distracted. It could well be that they multi-task a lot because they are so distractable and less able to focus.

A recent rigorous study of this matter evaluated a group of study participants, divided into those that were heavy multi-taskers and those that multi-tasked only infrequently. All participants were probably at the higher end of general mental capabilities, given that they were Stanford college students. Each participant was tested in a series of thinking tests to check for any difference in the way the two types of people processed information and disciplined their attentiveness.
  
Heavy multi-taskers were less able to sustain focus in the presence of distractions. The researchers concluded that light multi-taskers "find it easier to attentionally focus on a single task in the face of distractions."

The study did not directly examine ability to memorize, but there surely must be a significant difference, given that memory formation is enhanced by sustained attentiveness and focus. There may be some undiscovered benefits of multi-tasking, but memorizing cannot be one of them.

Nor is intelligent thought likely to benefit from multi-tasking. Several studies that I have summarized elsewhere show that intelligence correlates with working memory capacity, which under the best of circumstances is limited and easily over-loaded by multiple simultaneous informational input.

The study did not test whether distractibility is a cause or a consequence of multi-tasking behavior. But the data clearly support the notion that people whose work or study requires concentration should not multi-task.
There is also the potential problem that frequent multi-tasking could be teaching the brain to become more distractible.

Source:

Ophir, E., Nass,  C., and Wagner, A. D. 2009. Cognitive control in media multitaskers. Proceedings National Academy of Science. 106 (37): 15583-15587. doi/10.1073/pnas.0903620106