how do i know if a celebrity is online twitter?

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Next to the tweets it usually shows how many hours or days ago they posted their tweets. So you can just tell when they are on there, like some days they will be posting loads of tweets one after the other like every hour or so. Some celebs are on Twitter almost every day, posting their latest party photos or photos of when they wake up or play golf or whatever. I think it depends on their personality, some of them just "take to" Twitter and some just aren't really interested in it.