This year marked the start of what looks likely to be sustained decline of what had been the most popular social networking sites. 

【翻译】今年，最受欢迎的社交网站似乎开始持续下滑。

【结构分析】

This year （主语）marked （谓语动词）the start（宾语） of what looks likely to be sustained decline （of + 名词性从句，充当start 的定语）of what had been the most popular social networking sites.（of + 名词性从句，充当decline 的定语）