Well I guess it matters how important it is for you to get all the users interested in your service or a part of them. Right now there are two categories which you are definitely missing on: 1) People who don't have a Facebook account. 2) People who have one but are more privacy-concerned. How affected you are by these two categories is defined of course by the target audience (demographics and social status for example), but it is pretty hard to draw conclusions without alternate methods of registration (for I know those 500 registrations came from 1000 interested users).