Instant chat is currently only available to Pro companies. This means that a freelancer cannot start a new chat with you - but they will be able to see and respond to a chat you have started with them.
They will be able to see your 'online' status and reply via instant chat within 24 hours. After that, they can also reply via their TM inbox.
Other than those freelancers that you are chatting with, no one else will be able to see your 'online' status.