Many are just gateways to larger agencies, so check this before joining too many. Steer clear of those who have printed catalogues. Many of the girls on their pages have been in catalogues sold by the thousands. Keep your eyes open and you'll get a feel for the one's to avoid. They are unlikely to be in the business of truly trying to successfully match couples.
Agencies are in business to make a profit.
They run advertising in foreign countries to find women interested in seeking American husbands. They invite such women to write to them describing themselves, the kind of man they would like to find and to enclose a photo. They do not charge these women anything for this. They sort through all the letters they receive and publish catalogs at regular intervals, generally every two or three months. Many also publish on the Internet.