Hi there!
When I sign up with a new networking site, I always use the same address and have updates emailed to me as often as necessary. This way I do not have to visit the site several times a day.
Every morning I open my email and see if any of my networking sites emailed me with DM's etc. If so I give a quick looksee and move down the list. I usually only give this about 30 minutes - so I pick my favorite sites. (This one ofcourse, Facebook and Twitter - although there are tons of them that I like!)

Yep, I'm a nerd and happy to admit it!
Then I carry on with my day to day stuff. On my slower days I will check my email several times and respond to anything I find needs it. On busy days I try to check my email again for about 30 minutes just before or after lunch.
And again, just before my work day is over I mull over the email again. This time I only spend about 5 or 10 minutes. At the end of the day I am only looking for lose ends I might need to tie up or urgent requests I need to respond to. I leave the others for the next morning.
Once a week (usually Wednesday) I clean out my email box and visit & update all of my networking sites. I usually try to do this on Wednesday's but I must be honest and say it happens on Saturday a lot!
Hope this helps!