It can be dangerous anywhere. I'll try to keep this short....
1) I know a woman that met a man at church. Church helped them with the wedding. Two weeks later he tore the rings off her fingers to sell to buy dope. Eventually she had to leave the state under an assumed name because of him.
2) I know a woman that met a man at the gym.....she dated him twice. The very next woman he met (2 weeks later) he killed.
3) Met my first husband in school. Married after knowing him for 7 years. He ended up being a drunk and a wife beater.
I met my current husband on Match.com 8 yrs ago - been married almost 6. He is the best thing that's ever happened for me!!!!!
Now some advice on HOW: Select well, INTERVIEW them before you ever agree to meeting...talk for a long while and I mean for a month or two. Take notes when talking. You won't believe the men I talked to that had stories one week and forgot what they'd said the next. One said he loved fishing but 2 weeks later he told me he hated fishing. I even had one man tell me he was 5'11"....if he was over 5'3" I'll eat my hat. He accused me of being prejudiced against short men. I informed him I was not but I WAS prejudiced against liars! LISTEN to them, don't just be flattered. They'll pick up on what you say and try to say what you want to hear. If the hair goes up on the back of your neck, listen to it....that's your guardian angel.
The casual ways most people go at meeting someone online is scary. The "hook-ups" for just meeting and having a drink and then sex IS scary and dangerous. If you're serious about meeting someone nice, go at it like you were looking for a job.
Good luck!
BTW - I only met one druggy in all the dates I had...AND I didn't, as far as I know, meet a murderer. :) :)