If its a "missing" or a "long lost relative"?
You need to go through formal channels, but asking them to answer you back will take some time and perhaps to even personally go to their offices a couple of times with a formal request letter.
To trace anyone in the Philippines, a relative or a lost loved one provided you know their real and complete name. You may ask the NBI (National Bureau of Investgation) which keep records of any Filipino who has sought a job. Local Employers require an NBI clearance, a certificate that attests that the applicant has no criminal record. So therefore, if your relative has gotten a job or was once employed, her name will show up in their database, unfortunately which includes all her namesakes.
Second route would be the COMELEC, provided your relative has ever voted in any national election.
NSO (National Statistics Office) registers every birth recorded at any municipality in the Philippines. This includes late registrants, those who were never officially filed or registered by their mothers during the birth year.
Problem is, if a person doesn't want to be found, you'll never really find her.
For adoptees, I suggest to talk to your adoptive parents to release any further info they might have withheld like the last known address.
If your desperate, you may try to go on air and do a classic "panawagan", a plea for the missing love one to show up.
As for Missing Kids, you may try calling the DSWD or RAC or the TV stations for help.
If you are merely looking for that Pinay who stood you up on a date or for that visa money, chances are you may find her on Friendster.com