One online self per service is enough. I can't recall a recent situation where I felt I was acting differently in public depending on who was around. Certainly not differently enough that when I'm sitting at my computer I'm going to try to recall which persona I want to wear at which time. Perhaps I don't get out enough.
Ours is a nosy, judgmental species. Unless you can rely on being self-employed on your own farm for the rest of your life, you are someday (while you're seeking a job or an apartment or a partner) going to have cause to regret having put your True Name on a controversial idea. This is why HN gets substance while Facebook gets cheerful pablum from me, and I'm undecided about Google+.
It's not as extreme as that--consider just the case where you want to share things with work friends that might not appeal to college friends. You're not acting different; you'd show the college friends if any of them asked, but you know they're bored by it so you want to keep it out of the timeline/tweets/whatev that they see.
