You'll want to do a little research these days regarding the services each provider uses. Google made a stink a few years ago to go after this type of link building and to reduce the amount of duplicated crap they were indexing. They started recommending no-indexing and canonicals I think, and the press release companies tried to get around that by only keeping press releases alive for X amount of months, and then deleting them. It's not as great of a link building method as it once was.