Next month's election to the European Parliament will be a test of a phenomenon that has roiled US and British politics: the use of social networks to promote divisive issues.

Anti-immigrant parties make a disproportionate noise on social networks, helping to push their cause further up the list of voters' concerns. That may help such groups to make gains in the election, but it's far from clear they can turn that momentum into a cohesive coalition in the European Parliament.