If you have an addictive personality then you probably should be very wary of using websites like this, I have facebook and check it for maybe 10 minutes every day or two, I couldn't use it for longer because it's so bloody boring.
I check it a couple of times a day, but never for more than a minute. I don't get many notifications. Email is much more important to me and through trial and error I've set things so that I get emailed about important things, and nothing else. That said, it would be very hard for me to go for a few days without having Gmail open.
That's more of a necessity rather than anything else, though.
Once I got rid of my fb bookmark I immediately stopped checking fb so frequently. I almost feel guilty for not keeping tabs on it these days.