They come from people like Igor Strelk[ov], a national folk hero in Russia who commanded Russian mercenaries fighting in Georgia and later Ukraine. Some of these people might be slightly exaggerating Russian casualties to get the country riled up and pressure Putin into retaliating, but you can tell that Russia definitely took a huge hit based on the rage expressed by their troll army across the web, and the way those trolls celebrate the loss of a single Israeli fighter jet as if it was the liberation of Leningrad.
Strelkow was a national hero when he started the fighting in the Donbass against the Ukrainian fascists. Then he came back to Russia after this, he ended up in a quite dubious environment. I have considered him reliable at that time but stopped to trust him during the time of the Debalzewo operation in the Donbass war. At that time he was already out of the Donbass, but I thought he had yet enough inside information. But, not, all his predictions failed. In fact, in some other company, he could have become an important political player, but he failed. I would not think that he openly lies, I think he honestly believes this information.
The point of the Israeli fighter was not that it was successfully shot down - but that Moscow has given the ok for shooting it.
http://blogs.rediff.com/mkbhadrakumar/2018/02/12/israel-gets-bloody-nose-in-syria-what-next/ gives a nice summary what it is about. This was prepared step by step before, Israel could have understood that this will be the next step, but did not want to understand, now they have, maybe, understood the point.
BTW if Israel wants to wipe out half or more of Syria's air defenses, there's really nothing in the way to stop it.
Russia can only hold Israel back when it's able to simultaneously address Israel's security concerns, otherwise in terms of regional forces the IDF is many times bigger than the small Russian presence next door, and it theoretically has more than enough nukes to take the commies down with it if they want to go that route.
Feel free to believe this. We will see. There were over 100 such Israeli attacks per year, so, we will learn in short time if something has changed or not.
My guess is that Israel will not stop everything, but restrict itself to less dangerous things - cruise missiles from Golan Heights or rockets shot from planes over Lebanon, where they feel yet safe. This will be accepted by Russia for some time, but not forever.
Forget about Israel attacking the Russian airbase. Even more, forget about nukes, they will certainly not be used against Russians, the retaliation would be the end of Israel with certainty. Of course, Russia takes into account Israeli interests.
The more dangerous thing is actually the increasingly irrational behavior of the US. Looks like the warmongers have won and taken over the White House, there are some signs that one has to expect escalation toward a new Cold War. The US attacks against the Syrian troops are one such sign. The revival of the last hospitals and the gas attacks propaganda another. So I'm not optimistic about near future.
In Northern Syria, the Turks slowly advance from several sides toward Afrin.