If your status update is posted to only Friends, and no one is tagged in your post, then Joe's friends will not see the post or the Like on the post unless they are also your friends. This is (c) if there is no tagging or mutual friends.
If it is posted to Friends and Joe is tagged in the post, normally that will make the post and Like visible to Joe's friends as well. Joe's friends may see it in their News Feed, although they may not (Facebook determines what it thinks is most relevant for your News Feed, especially if you have a lot of friends). Joe's friends may be able to see the post on Joe's profile/timeline, although Joe may have removed it from there or may have changed this using his Privacy Settings.
If a post is not visible to someone then that person will not be able to see Likes on the post either.
For now the best way to follow the tag is to either search for or click on an existing post with the tag, which will bring you to the Hashtag Page
According to the FB blog post announcing the features, the current rollout of the FB hashtag will allow:
Introducing Hashtags on Facebook
Starting today, hashtags will be clickable on Facebook... When you click on a hashtag in Facebook, you'll see a feed of what other people and Pages are saying about that event or topic.
There will be a hashtag feed that you can currently search and/or click on to, but the follow feature itself hasn't fully been rolled out according to Facebook Gives Marketing Advice For New Hashtags
Facebook users will be able to search for a specific hashtag from the search bar and compose posts directly from the hashtag feed.
The Facebook hashtag system will be slightly different from Twitter since it will still be subject to the same privacy restrictions. From TechCrunch's Facebook Announces Searchable Hashtags, Promises More Features For Following Public Conversations
A company spokesperson told me that from a privacy perspective, hashtags will work similarly to Facebook Graph Search. In other words, you’ll only see the comments that you’re authorized to see. So if I include a hashtag in a friends-only post, then my friends will still be the only ones who can see it in a hashtag search.
Best Answer
Yes, they will get notified. Some individual may turn off notification for a new follower. In that case, no.
Now if you're wondering, Will they get a notification like, "arieljannai is following you."?
That depends on how many followers they got in the last certain amount of time. It can be from a few hours to a couple of days.
If they get a follower every once in a while they will get the notification:
"arieljannai is following you."
But if it's a popular public figure, they may get a notification:
"John, Jenny and 30 other people started following you"
Where your name may not even be shown. But they can click on the notification or check their follower list later in time and will find you there.
When you un-follow someone they will not get a notification. Neither if you re-follow a friend. (source)