To "hitchhike" via internet is definitly much more safety than to thumb up a ride. But there is still possibilities to increase safty.

• Contact your driver on his/her mobile. With a telephone call you get a first impression how your potential driver is.
• At least you should call him/her once by yourself to make sure that his/her mobile number is right and exisiting.
• Inform other people about your ride. Tell them the ride data e.g. time of departure, way, approx. arrival time, etc. Best would be to inform third people about the mobile number of your driver as well. 
• Appoint a Meeting Point where a lot of people is around e.g. railway stations, petrol stations or motorway service areas.
• If you appointed a ride, all details are checked, you had a good first impression of your driver and this good impression is gone when you see him/her first time, don´t step into the car. Safety should have higher priority then anything else.