Is there a limit on how many friends you can have?
Eric Barker stashed this in Diabolical Plans For World Domination
I had this very debate with David Weekly a few years back. He insisted that technology invalidated Dunbar's Number. My feeling is that the upper limit is based on there only being so many waking hours.
Technology helps in that we no longer have to hike to the next cave to see a friend, but it doesn't mediate the actual interaction.
I think it's possible to have many more people think they know you because of your Facebook/Twitter/Blog feeds, but that's a 1-way relationship. You don't know them. Simply having ambient awareness of their activity isn't friendship.
Time is a huge issue. Combine it with mental limits on how many people can be "top of mind" without some sort of formal schedule and I think you have much of the cause of Dunbar's number.