Hey everyone! I've been following this thread since its inception, but haven't ever commented. That said, I thought I'd jump in to offer help/answers about Twitter for anyone who needs them. I've been on Twitter for about 7 years and I actually joined Websleuths to follow a very Twitter-centric crime (Skylar Neese).
To answer overclockedgirl:
1.) Anyone can tweet at anyone else using the @ symbol followed by that person's username. It doesn't matter if you're following them or if they're following you. It also doesn't matter if they have a private account. They can still see your tweet (unless they have blocked you). Also, many users get emails when someone tweets to them in this way.
2.) You can click to follow anyone (again, unless they have blocked you). If that user does not have a private account, then they do not need to accept your request. You will automatically be following them. If the users account is private, then they will need to accept your request to follow before you will see their content on your feed.
Also, someone asked about hashtags a while ago and I'm too much of a bum to go back and look, but this should answer your question:
-The # (hashtag) is used to create trending, or popular, topics. When you click a hashtag, it will bring you to any tweet that contains that tag. It will not only include hashtags from users following a certain account, but all content throughout Twitter. So, if a tag exists, for example, saying #RIPTom, and you click it, it could very well give you tweets about any number of recently passed Toms, not just the Tom we're discussing here. Specific hashtags are often created for certain movements or events, so look for those. Vague hashtags will often lead you through mountains of unrelated tweets.
I hope this helps, and please feel free to ask any questions you may have. All the best to those who love Tom. I hope you find answers and peace in this tragedy.