If the online you are quoting from has paragraph headings numbered, cite the paragraph number using "para." For example, Smith and Volf (2009) argue for "a realistic ethics of the heart" (para. 7). Or Some argue for "a realistic ethics of the heart" (Smith and Volf, 2009, para. 7).
If the source you are quoting has neither page numbers nor paragraph numbers, count the number of the paragraphs down from the top of the article or page and format the citation the same way as step 1.
If the blog or article has section headings, guide the reader by stating the first couple words of the section in quote marks, then give the paragraph number. For example, Miller (2008) sees "gold where others see only a silver lining" (The Benefits of Optimism section, para. 5).