I have my facebook shop page and my pinterest boards linked to twitter so that anything I post to either will be tweeted. I share stuff from other shops, other sites, I'd say 90% of what I FB or tweet or pin about isn't related to me but to interesting links I've found, interesting shops, or I retweet things that look interesting to me that maybe others want to know about. I know people who retweet EVERYTHING their followers tweet and I think that's lame, personally. I don't want to flood people with stuff.
What other people say here is all true--don't make it all about you, tweet about things that are interesting, even mundane sayings and quotes, funny things. One thing someone told me was to avoid politics and religion or bad language--those are turnoffs. I've had people follow me who are politically or religiously inclined and I don't follow them. I'm not interested.