I think the multitasking question is a good one, but I think it is usually approached poorly. We are terrible at multi-tasking, and we are terrible at judging just how bad we are at it, that is true, but we have surrounded ourselves with informational objects and entertainment media for centuries, and managed to get things done. So why are modern technologies different?
I think for two reasons, one is that they are not, but that we end up discussing it in a sensationalized way that says "we are easily distracted and cannot multitask ergo this new thing is bad for us." I don't think this problem is unique to this time period, my 18th century readings are full of people worrying about the number of novels available (particularly to women) distracting readers from the 'important' texts from perfecting other skills from properly socializing, and warping their brains. Over our history we have successfully worked in rooms which contain clocks, calendars, unopened letters, books we have not read, books we have read an love, windows with views, paper and pens, and all manner of other things which might distract us, and still managed to get things done. In that respect our objection to these new technologies is expected and as in the past, we will develop a sophistication in out interactions, and a cultural understanding of how to use a technology, which will not distract us.
The second reason, and I think the much bigger concern, is that (pardon me while I spew iSchool cliches) now that we life in an attention economy, the companies developing and marketing these products have an interest in you being distracted by them, and so have little incentive to make them unobtrusive. Now, surely every clock maker hoped you would look at their clock, and every author hoped you would read their book, but their income did not depend on the number of times you did so. Many of our distracting technologies are based around a business plan which needs you to keep coming back. Google needs you to keep searching, and needs you to do so, not just instead of using Bing, but instead of stoping to figure it out yourself, instead of asking the professor, instead of.... In user tests they measure the success of their page layout based on how many times a user returns to make a new search. Facebook needs you to keep checking your page. It is in their best interest to make their content distracting, and to beep you when there is an update, and to structure your experience with them into something that will keep drawing you back. All the stupid 'games' for Facebook and phones need you to keep returning and so on.
One solution is to be aware of your relationship with a company (are you the customer or the product?) and to minimize your interactions with products which are selling your attention. Sometimes this means just abstaining from a "service" and sometimes it means paying the premium price for it and sometimes it means developing habits which set you up to succeed. If you can't stay off your Facebook when you are taking notes in class, disable your wireless. If you can't stop checking that stupid game, delete it. If your phone buzzing a beeping distracts you use an application like Tasker to customize when it does what, so that is is something you control, not something that controls you.
I think it is important though not to get too cynical about companies selling our attention though. Yes, some business are based solely on the idea of distracting you so that you will either be eyeballs for ad views or so that you will pay for content, but there are others who use that structure to give you a service you would not otherwise be able to afford. I don't think any of us could pay Google what they make on our web searches, and I for one am not willing to give up the service they are offering me.
(This essay brought to you by Kat's over abundance of words, as mentioned in the Random thread.)