Should I be concerned about plagiarism with beta readers?

When I wrote my first YA novel, I didn't receive any feedback before submitting it to agents. Big mistake! The second time around, I really want to take advantage of beta readers and critique partners that can be found online. Trouble is, I'm a little reluctant to send my manuscript to complete strangers online. This thing is precious to me. The idea of handing it out to strangers I've only met online is almost as scary as the idea of sending my social security or credit card numbers to someone online. How do I find reputable betas? In forums, such as Absolute Write, people list their beta services, but you don't even see their full name or photo. How do I prevent against plagiarism or other funny business?