There are a ton of Twitter. There is a site something like pornexectweet.com, sister site to PornStarTweet.com which lists all the porn stars who are on twitter. Anyway here are a few adult companies off the top of my head on Twitter.
My own of course :P
http://www.Twitter.com/KlubKelli
Amateur District movies
http://www.Twitter.com/AmateurDistrict
Skinworxxx
http://www.Twitter.com/MrTeagan
Mallcom
http://www.Twitter.com/Mallcom
http://www.twitter.com/xbiz
http://www.twitter.com/avnawards
http://www.twitter.com/xcritic