Some sites offer the ability to perform complex searches for people which may be compatible with yourself. For example, they may share the same interests as you, or you may meet a requirement which they need you to meet - such as "you do not smoke" or "you have dark hair".

The process is achieved by collecting as much personal information about you as you are prepared to supply, don't forget, the more accurate the information, the more chance you have of the system providing you with contacts who may be compatible.

Remember that supplying the information need not be viewed as in anyway dangerous - the reason for this is because the personals sites and matchmaking systems they run operate in a very secure way - which means that as the information is sent over the internet it is encrypted.  Also - at the other end - when your information reaches the database it is protected by layers of technology which attempt to stop the hackers getting in and accessing the information you and other members have sent.

So don't be too concerned about security - but at the end of the day - the matchmaking sites do store the details - so if there is something you really want to keep private - then simply keep it that way - and do not send it in with your member profile.  It doesn't really matter except for the fact that when a search is performed, you may not show up in the results because the information you kept to yourself may have been a requirement of the user.