It's quite simple. "Multiculturalism" and religion have little to do with this.
Population of Sweden 1976: 8.2m ----> 2004: 9.1m (11 percent increase)
Number of employed in total during same period: 4.0-4.2m (stagnation)
Most recent youth unemployment (age 15-24) figure is 27.2 percent (April 2013).
Employment took a big dip in the early 90's with the recession. After recovering from that it was a booming economy in the early 2000's until the international financial crisis hit in 2009, but in reality employment had only clawed back to the same levels as before the recession. So over a period of more than 30 years we've had completely stagnant job growth. There's also been substantial cutbacks in welfare during all of this.
But there's more to this. It has been official policy (although not openly stated to voters

) to drive down real worker wages in Sweden since about 1985. This was uncovered in a TV-documentary ("Lönesänkarna") last year where journalists interviewed the officials responsible for implementing these policies -- public office holders, union representatives, economic scholars etc, in cooperation with prominent leaders in business and industry. Perhaps you'd expect this from right wing politicians but amazingly enough this was driven through by the Social Democrats. They openly admit that the goal was specifically to lower the wage share and increase the profit share in order to allow capitalists to invest into new businesses and job growth.
None of that shyt happened, except for workers making less money. The average monthly worker salary in manufacturing today would be 10k SEK higher at 35000 SEK if the trend up until early 1980's had been allowed to continue, which had essentially kept pace with productivity for more than 50 years. Wage share of revenues is now down to levels from the late 19th century. Unsurprisingly capitalists with their newfound gains chose to earn an easy interest from international capital markets instead of getting their hands dirty and actually creating someting new. That shyt takes real work and effort
The documentary received the silent treatment in news media.
In summary, trickle-down Reaganomics took hold here as well, and you can of course add automation and offshoring to the mess which is significant in Sweden's export-oriented economy. We're now reaping what we've sown, same as you. Anyone believe people start rioting if they are gainfully employed?
** Not enough jobs to go around. Lower wages --> lower demand for goods and services --> less work.
** Segregated communities, as daJungle has already explained well.
** Antagonism ensues -- nasty immigrants leeching benefits, stealing jobs / racism holding back people -- with Sweden Democrats fanning the flames.
** Civil strife occurs when people don't have shyt else to do with their time.