Online-only ("OO") relationships† are fundamentally different from real-life ("RL") relationships, even beyond the obvious differences like lack of physical contact. In both OO and RL relationships, you can limit what you reveal about yourself to just what you think will be most flattering, but in OO relationships, that's the default rather than the exception. Depending on how the parties in an OO relationship meet, details like real name, gender, age, and physical ("meatspace") location may not be obvious, or may not even come up at all. Webcams are inexpensive and easy to use these days (often built into laptops and cell phones), but not everyone has one or wishes to use one, so personality insights that would normally be provided by speech patterns, body language, and physical appearance (height, weight, and more tellingly, clothing choices, hygiene, and "decorative" touches like tats or makeup) may be absent or a limited to a few carefully chosen photographs.
Is it possible to develop genuine affection for someone you've never seen in the flesh? Is it the person you admire, or their words? Intellectually, you know that they might not have given a complete or accurate portrayal of themselves. ("On the Internet, nobody knows you're a dog.") Emotionally, it's not easy to resist companionship, even in electronic form. And if you do find yourself attracted in a friendly and/or romantic way to someone you've never met, at what point—if any—is it appropriate to start signing your emails, "Love..."?
† By which I mean relationships in which the parties have never met face-to-face and have no specific goal to do so. As distinguished from "long-distance relationships" in which the participants plan to meet or have already met in real life, and plan to do so again, but in the meantime, stay in touch by phone, snail-mail, and/or the Internet.