I think the above posts adequately address the issue of GMs/other players having to take control of absent player's characters in order to keep the plot going. It's relatively common in long running RPs, is something I have experienced myself from both sides, and I haven't really ever seen it be abused by those stepping in to fill the absent player's shoes.

But there's another facet to this that I think we should all bear in mind when it comes to posting OCs on the internet, which is: None of this Copyrighted, people can and will take your ideas and do with them what they may, don't get upset about it. In that sense, yes, your original characters are up for grabs.

Most often no one is taking 'your' ideas specifically, they're just aiming for similar vibes or have similar influences etc. But it will happen occasionally in ways which are little bit too specific to be coincidences. Case and point, a player I was in an RP with posting a character in another RP that had the same class, appearance and name as my character in the RP I was playing with them. Bit weird, but whatever, we all take inspiration (see: borrow; steal) from various sources, and we're all putting this out there for anyone to read.

Anyway, that's relatively rare in my experience, just though it deserved a mention.