Just like email addresses, you can have multiple ones, and speak with everyone else (AFAIK). What changes is that you also have a timeline (like a mailing list that goes in a folder) for your local instance, and one for the instances it federated with.
As far as I know, a lot of people have different accounts for their different centers of interest.
That comparison doesn't map gracefully though, given that people don't "subscribe passively" to a single email account, and email users don't (outside of mailing lists) generally broadcast.
As far as I know, a lot of people have different accounts for their different centers of interest.