时间状语和地点状语的通常排序为，先时间后地点。例如，Come here tomorrow。He graduate from the school last year. 但有时地点状语结构复杂或为了强调地点状语，也可以将地点状语置于句末。例如，Come tomorrow to the room I have shown you just now.

所以，when I had seen them online before 通常的说法，had seen them before online则是强调地点状语。

还有一个原因将online置于before之后：因为see sb/sth online before 结构有歧义，online可以理解为宾补，也可以理解为地点状语。online移至时间状语之后，online就只能是地点状语，不可能是宾补了。