What's your join % from invitation e-mails sent by your members?

I currently have 1:124 join ratio from invitation mails sent by members which imho is really bad considering I have 1:33 ratio from on site visitor joins and from the social psychology research I read, friend invitation joins should have a MUCH higher ratio than joins from traffic received through commercial advertising.

Can anyone provide any comparative figures, or ideas and recommendations - e.g. should invitation e-mails be plain text or html?

Thanks