It's not uncommon, that's for sure. Personally I don't think I can do that, but I understand why others might be able to. Some people can fall in love with a person just through what they say online and how they interact with each other online. Falling in love with someone only through talking to them is almost like you're falling in love with their personality which is good. It's better to love personalities than physical appearances. Although some people do fake it when they're online, which is the reason that I don't really think I could fall in love online. But that doesn't mean that it's wrong for other people to fall in love with people online, it just means that it's different for everyone.
And if you fall in love with someone online and it happens to be genuine, then that's great. I believe that as long as the love is genuine, it doesn't matter how it happened. Love can come to us in many different forms, so I can't judge that.