The most popular social identities are Facebook, Google, Yahoo, and Twitter, but popularity differs by user segments and website categories. Users are most likely to log on to entertainment sites via Facebook, but Twitter for news sites, etc. 

Given the data that is available to companies if their users subscribe using one of their social online identities you wonder why there's still normal subscription processes being used, especially as the social identities probably have better data quality anyway (i.e. you want Facebook to have your real/main email address).