The Electronic Frontier Foundation has posted about the "Six Heartbreaking Truths About Online Dating Privacy." It's the usual story: once you load information about yourself in some online database, you lose control over it, no matter what they say. (Hat tip to Be Spacific for the link.)
If you think about it, it scarcely makes a difference whether the dating service is online or not. A brick and mortar dating service will have a networked computer database too; that's usually the whole point of the business. If they have internet access, that database is a rich target for cyber-criminals to hack.