Huh. My understanding of Twitter is minimal, what with having all of nine followers and just tweeting as though nobody reads it anyway. I thought there was a notification for tweets with your handle in them. If not, that would certainly reduce the chances of it getting noticed!
If you mention him i.e. write @michaelshanks then he will see it. It will go to his notifications or mentions. Problem is he probably gets tons of them. So he probably misses a lot or pays little attention to them unless he's met them.
Seems to me he is more likely to re-tweet/reply to people if he's met them at a convention.
He may have a #savinghope search on going in whatever app he uses to access twitter.
Laura, I don't know if thats a rule of his or not but he probably gets at least 10 replies to every tweet he sends plus any extras like people asking for spoilers for series 3 of saving hope or asking if there is going to be more stargate or any other question that pops into his head.
Reading his timeline, it seems to me he replies to or retweets stuff from people he's worked with, or people who are promoting a show of his or fans he's met at conventions. I don't know if that is a hard and fast rule or not.
Also lately he's also re-tweeting/tweeting stuff to do with the Vancouver school strike.