Here is a chart that compares the 3 services:http://www.isogg.org/wiki/Autosomal_DNA_testing_comparison_chart
I am winging this so I may have to make some changes. I began with 23andme when they used the V2 version of the Illumina chip, when they switched over to the V3 version, it required an additional sample. Some SNP's on V2 were dropped and many added, with an emphasis to add SNP's related to health. Existing 23andme customers could upgrade and have both sets. I think My 23andme raw data file has around 1 million SNPs tested.
FTDNA used the same chip as 23andme but removed health related SNP's, like Parkinson's for example, that might cause a problem with the FDA. The remaining SNP's 23andme would characterize as traits. The FDA is not concerned with Traits, like wet or dry earwax.
AncestryDNA's used the same Illumina chip and I believe removed the same health-related SNP's but added ancestry related SNP's. Ancestry Related SNP's have more to do with Ancestry Composition and less to do with genealogy.
23andme then upgraded to their latest version V4 on a new chip. They greatly reduced the number of SNP's and focused on those that also added value to their focus on health.
Each of these 3 companies actually focus on 3 different markets. FTDNA focuses on the genealogy, AncestryDNA Ancestry, and 23andme Health.
I can't tell you how many people at FTDNA have expressed some negative attitude because I manage 12 accounts, who they do not have direct contact with. Something that is quite common and expected on 23andme.
I can't tell you how many people at AncestryDNA and 23andme who have expressed surprise at finding a list of DNA Relatives. My AncestryDNA cousins tell me they bought the kit or where given the kit so they could find out if their Ancestry was what they expected. This is part of a major advertising campaign. 23andme cousins bought or were given a kit so they could get health results or so they could participate in some illness related sponsorship. In other words, a single 23andme program might subsidize all or part of 30,000+ kits and not inform them about the other features.