It depends on the nature of the relationship I suppose.
You have to be careful with people online because there's no way of knowing whether they are who they say they are. But if you've spent a lot of time chatting with someone, even if only online, you can get a better idea of who they probably are. If you exchange emails and phone calls you get a better idea yet.
There's obviously a lot of scope for people to bend the truth if not outright break it online, although sometimes it's worth asking what someone has to gain by doing so. In the context of a romantic relationship there's an obvious potential to scam people by pretending to be the lonely heart they are seeking, drawing them in and then scamming them. In the context of something more akin to a friendship there's so much less to gain. I used to use another forum similar to this one before it shut down and am still in loose context with some of the people I got to "know" there. I've never met them in person but the communications I have had with them has been enough that I'm inclined to believe they are who they say they are. If they suddenly started asking me for money I'd be a lot more wary.
Romantic relationships seem to have as much chance if they start online as if they start offline. Chances are several of us know people who have met long term partners on a site like Tinder.