Trust must grow. Of course, you can also get to know each other well over the Internet and a friendship can develop. But to be able to really trust someone fully, it would not be enough for me to know someone only online. It's always something else when you know your counterpart in real life as well and maybe even be part of their everyday life and know what really happens. Most people are also honest on the Internet. But there are also other people who pretend to be someone else than who they really are. If you only know someone online, even after years of intensive contact it is sometimes hard to tell if it is really a real person with a real story or a real fake.
I met my ex-wife online, as well as a few other ex's (clearly they didn't go very well) my ex-wife and I were very compatible, except she hated where I live, and I hated where she wanted to live - we split so we could both stay sane and live where we were happy. Fiancee after her (we never got married) I met online and we were together for 7 years.