Loud Whispers, using this source:
http://www.vox.com/2015/11/19/9760284/isis-historyI'm guessing that your 800 figure comes from the number of ISIS fighters in the battle for Mosul in June 2014.
Assad ensured that (a) there would be a civil war instead of peaceful protests, by immediately attacking protesters, and (b) that the civil war would be dominated by islamic extremists, by attacking the most moderate / secular rebels and ignoring the most extreme ones, and also:
In amnesties issued between March and October 2011, Assad released a significant number (exact counts are hard to know) of extremists from Syrian prisons. Hof called this an "effort to pollute the opposition with sectarianism": Assad gambled that if his enemies were Islamic militants, then the West wouldn't intervene against him.
(ISIS, in 2012-13, also freed "somewhere in the neighborhood of 1,000 inmates" from Iraqi prisons, including "many terrorists [that] elite US military forces caught over the years." Of course, that has nothing to do with Assad.)
Once ISIS was declared (from AQI) and in Syria (2013), Assad avoided attacking them, and ISIS concentrated on attacking the other rebel groups, including Al-Nusra, to take land that had already been taken from Assad's forces.
Once ISIS was declared a caliphate after the fall of Mosul and the blitz across Iraq (2014), recruitment went way up:
Establishing a caliphate had long been the goal of the entire jihadist movement. By declaring that he had actually created one, Baghdadi gained a huge leg up on al-Qaeda in the struggle for global jihadist supremacy. Since then, ISIS has "succeeded in attracting far, far more recruits" than al-Qaeda, Will McCants, the director of the Brookings Institution's Project on US Relations With the Islamic World, told me. This has also has allowed it to gain a following among foreign terrorist groups, with major ISIS franchises in Libya, Egypt's Sinai desert, and Nigeria.
Even so, they've been losing ground and suffering enough setbacks lately that their image of being winners (to use a Trumpism) was at risk, so they resorted to dramatic Al Qaida style suicide attacks in foreign countries, including western ones since western citizens tend to ignore news articles about them in the middle east (and then apparently blame the media for not covering them :V).