If you haven't met the person personally (as in physically meeting them) and you want to have a relationship with the person then yes it can happen and it is becoming more common.
HOWEVER, there are a few risks that you need to be aware of.
- Since you don't actually meet, the chances of cheating is raised and this is where trust needs to be - a strong bond of trust.
- You can see and talk to the person (well write to or whatever) but you can't actually touch them. Shame I know.
- You do need a lot of communication like any other relationship because otherwise it can fall as easily as a relationship with people who can meet.
Of course, if the person you live with doesn't live too far away (a city away, or a county away etc.) you could meet the person, but if it's an international relationship (i.e. the two people are in different countries) then that's harder.
As said, it is becoming more common as social networking sites like Facebook are becoming more popular.