It's okay, but I don't recommend it personally unless you both have a means of somehow meeting each other at some point within the year it starts. Otherwise it often feels like you have an imaginary boyfriend/girlfriend, and it also increases the temptation to cheat because there's often no way they'd ever find out. And even if you read that and go, "Well I'd never cheat!" the sad truth is most cheaters feel the exact same way before they eventually screw up, so don't completely write yourself off as someone who wouldn't do that. We all make mistakes. And naturally if it increases the risks with you, it does with him/her as well. So anyways, I'd just say be careful. I've had several online relationships in the past and while some of them left me with some great friends that I still talk to even though the dating part ended, some of them ended very very badly and I made some new enemies online, some I deserved to have, some that were just jerks. And while it is easy to avoid online enemies by simply staying off the internet, there's always a sadness in seeing someone who used to care about you or seemed to, turn into someone who despises you.
The only other piece of advice I'd have is if you decide to do this, don't start an online relationship with someone who asks within the first 3 sentences of meeting you, "You wanna date?" just like you probably wouldn't in real life. Only do it with people that you've already established some sort of friendship with already and if it grows into online dating it tends to go better, from my experience anyways. The ones I got to know and had a pretty solid friendship with went a lot better than the ones I barely knew.
And thanks to the scaremongering media, you know the obvious rule by now. Be careful on the internet, there's strangers lurking on there who will blah blah blah... you know the drill.