Right. So here we have "the myth of the ten thousand," as I like to call it. According to Nils here, there are ten thousand Palestinians (including women and children) who are just sitting in Israeli jails as we speak. There are many other Huffington Posters who will repeat the myth of the ten thousand in many different places and contexts as well.
Unfortunately, this is again another transparent lie. You can look at B'tselem's website and you will find the actual statistics. There are currently (as of B'tselem's last update) about 237 Palestinians being held in adminstrative detention. Not even close to ten thousand, and if look at the stats on that page some more, you will see that most prisoners in AD are released after no more than a year and a half. It doesn't even say which were arrested for "being peaceful" (thought according to the HPers that would be all of them) and which were not.
Now, admittedly this doesn't include actual prison. B'tselem has stats for them as well, and that includes much more, about six thousand. But again if you look you will see that pretty much all of the six thousand were either convicted in a court of law or are being held until conviction. The number in AD is a fraction of that. Usually, though, when the HPers use the myth of the ten thousand, they are referring to administrative detention, not terrorists who have been convicted.
So I guess the question now is where did the ten thousand number come from? My theory is that since the start of "the occupation" ten thousand Palestinians have passed through the system, and were released again (or not). Regardless, to claim that Israel is holding ten thousand Palestinians "hostage" is simply yet another half-truth.