Iteractions some you can do?
Http://developers.facebook.com/policy/ here: "If you will, or plan to exceed each threshold following please contact us as there may be additional terms and conditions are subject to: (> 5 m Mao) or (> 100 m API calls per day) or (> 50 m impressions per day)."
You can also perform these requests in groups of up to 20 batch queries. Http://developers.facebook.com/docs/reference/api/batch/
Is there a limit to the number of members can do that for?
Because I don't know.
A better solution is to simply write to the user's news feed (& not the wall of each member)?
Yes! Is less spammy way. I usually block these sorts of posts and report them as spam
Is there a way to write the query to graph API signaling only once but the "to:" field sent each member you want to publish???
I think, but I noticed that people complain not working anymore.