I will keep a lookout for your bike.
It is sad that bike theft is so rampant here, and i worry about it almost constantly. For future SF travelers you should know that the area around downtown and especially the library is crackhead central, and they will steal anything and everything from your bike that is not locked down, or, as in the OP's case the whole bike. Either use a heavy u-lock or park the bike inside somewhere safe (maybe the hostel can work something out) and leave it there while seeing the sites.