I bought my iPhone last year and since then my wife has been telling me off for being on it all the time. 3 weeks ago she got herself an iPhone and now she's exactly the same.
I think it's the fact you have access to everything instantaneously rather than having to switch on your PC/laptop, wait for it to load up and for software to open. Before I had my phone I very rarely used things like Facebook or email (except from work) but now I'm on them all the time, plus the fact you can just dip in and out of games for a minute here a minute there.