The greatest challenge I've faced as an over-50-year-old is wrapping my mind around the non-linearness of social media, and Twitter is the epitome.
If I can't figure out why someone has followed me, I wait a couple of days and then ask. My photo is DCM's face so I'm not surprised if it's a horsey person or business. Otherwise, I can connect the new follower to something I've tweeted to someone s/he follows or labeled with a hashtag.
FWIW, I read somewhere that Facebook is going to add support for hashtags.