I used to be very frustrated by this. I have a shortlist of about four sites (gmail, reddit, HN, google reader) that I keep reloading obsessively. Some times I would catch myself closing reddit, just to open it again. Entire days could be wasted in this fashion.
Recently I've stopped doing this as much, I half suspect it's a mixture of exercising more at the moment, going out in the sun and the fact that I don't think there's much new material out there. Maybe I've simply become bored of it.
So much of what we read on these social sites and on our RSS feeds are just people vying for our attention. It's like being in a room with people all shouting at you sometimes. Newspapers want your eyeballs, bloggers want you to read their opinion. CLICK MY ADS!
You're right, it really does trash your attention. Now that I've eased my use of it I find myself working away and then being surprised (and a bit disappointed!) that it's going home time!
Look at the top entry here on this site at the moment - it's about a guy responding to some other guy about some Apple stuff or other. Do I feel more enriched by reading this or getting on with some work? Does what he said have any bearing on my existence? Do you really need to read articles like "10 jQuery widgets that will change your life!"?. Do you really need to read the 200th article on the oil spill? Is reading it going to magically clean it up?
So in response to your actual question I would suggest working out some sort of routine - only spend an hour or so during lunchtime and after work so you have set times you should be working. Just have a quick skim over the first page of these sites to see what's going on. Longer articles can be saved for later and read when you really don't have anything better to do. It'll still be there.