Yes. Yes they can.
I met my first real girlfriend on the net, was with her for almost 4 years, 3.5 of which was long-distance. Why did we break up only a few months after finally moving in together? At the time, we were of ages when people do a lot of their "growing up", and personaly change a lot. Three and a half years at just that point in a person's life can make two people not "growing up together", grow apart.
Really though, it was a very mutual end when all was said and done, but the relationship was wonderful and i'd do it again if I found the right woman. I wouldn't recommend it for just anyone, however, because it can be very taxing. It's hard, there's no two ways about it- but it's also ultimately very rewarding, and who can argue with that? lol
(Plus, there's nothing quite like the energy level of the sex you have when you only get to see each other twice or absolute max 3 times in a year, for usually a week or two at a time. Insane.
)