Is this plagiarism or just a bad practice? When I first began applying correct citations and referencing, I followed methods such as placing a citation at the end of the paragraph if I am paraphrasing for one person and not following a citation for each sentence. For example:


  One can use this website to write about various activities and interests such as music, art, science, etc... and one can also get in touch with the founder. However, to use the website, a person has to register and pay a fee. The website founder is known for his IT expertise. More people are joining nowadays than ever (Smith 2010, pp. 1-2) and those people can be great contributors to the website. Contribution is an important aspect of online communities (John 2014, p. 5).


Now I am reading about how each one complete sentence should be cited instead, and by using lead-ins to help the readers. In my above example, I want my readers to understand that the first 4 lines are Smith's ideas and the last 2 lines (from 'and those people') are John's ones. Now I realize that such a practice is a poor one as it may confuse the readers.

It should be something like (using lead-ins or ibid. for more readability, of course):


  One can use this website to write about various activities and interests such as music, art, science, etc... and one can also get in touch with the founder (Smith 2010, pp. 1-2). However, to use the website, a person has to register and pay a fee (Smith 2010, pp. 1-2). The website founder is known for his IT expertise (Smith 2010, pp. 1-2). More people are joining nowadays than ever (Smith 2010, pp. 1-2) and those people can be great contributors to the website (John 2014, p. 5). Contribution is an important aspect of online communities (John 2014, p. 5).


My question: Can what I did in the past be considered as plagiarism (as in not making it clear whose ideas are for whom (like readers may think that unreferenced sentences are mine) crediting wrong persons, etc...) or just a bad style of writing? (Field is business and using, obviously, author-date system)
Referencing your source, but in a somewhat ambiguous way, does not open you to charges of plagiarism.

It's good to want to get this right, and the first example you give is slightly less clear than it could be.  However, given that you have referenced your source right in that paragraph, no one is going to think you are plagiarising.  This is an minor issue of style and clarity; it comes nowhere near academic misconduct.

The repeated references in your second example remove the ambiguity, but they are rather annoying and distracting to the reader (though it is less of a problem in more concise style conventions).

To cite a source for a whole paragraph or section of text, add an introductory sentence and then put the reference there.  

This will make it clear that the source was used for the whole discussion, without needing to repeat yourself.  It also tends to be clearer, better writing in general. For example:


  The blah blah blah website is one example that illustrates this phenomenon (Smith 2010, p. 1-2)  One can use this website to write about various activities and interests such as music, art, science, etc... and one can also get in touch with the founder. However, to use the website, a person has to register and pay a fee. The website founder is known for his IT expertise. More people are joining nowadays than ever...


Or something like.


  Smith et. al. provide a description of the website functionality (Smith 2010, p. 1-2).  One can use this website to write about various activities and interests such as music, art, science, etc... and one can also get in touch with the founder. However, to use the website, a person has to register and pay a fee. The website founder is known for his IT expertise. More people are joining nowadays than ever...