I've done it only twice, and I've had both results.
The first time, he and I agreed we were "on" and it was going to be an awesome relationship. We met the week before on a dating site and were emailing and IM'ing everyday, and he lived in the same area. And we just really hit it off intellectually and physically.
Five days of silence after that one date, then a defriending on FB the day after Valentine's Day, no explanation whatsoever.
Later met another online, emailed two months, met in person for a five day long date (since he lived in another country), had sex that first night of meeting, declared that we intend to keep it going.
Eleven months after meeting online (nine months after the long weekend date), Master and I are still together and neither of us are opposed to anything permanent. But we're exclusive to each other, anyway.
It REALLY depends on the person.