Well I never would have thought so up until I met my boyfriends friend who met his wife online and they've been married for 4 years now and are extremely happy together. Just make sure you really know this person well enough to consider meeting them and maybe have a friend nearby just to be safe.
Yes, if you're over 18, have been conversing at least a month & 100% comfortable meeting them. But never meet them alone. (First time, anyway.) Meet them in a public place along with some of your friends. If they're a decent person, they'll be fine with that. If not, forget it.