If you social login is not working then please make sure you have insert your social info in plugins settings. 

Please go to 

1- WordPress Dashboard --> Settings --> Nextend FB Connect--> Have you insert Facebook APP info here? If not please put your info.

2- WordPress Dashboard --> Settings --> Nextend Google Connect--> Have you insert Google APP info here? If not please put your info.

3- WordPress Dashboard --> Settings --> Nextend Twitter Connect-->Have you insert Twitter APP info here? If not please put your info.

Thanks