I think, an author should wait to publicly announce his/her manuscript until the process of publication being done by the publishing authority. Since a preprint can be the reason for many risks regarding the publication process so, why is it??? Scientific researches require rigorous physical and mental endeavor along with time and money. Now, the rush of publishing preprint might ruin the worthwhile attempt (by the issue of similarity, pavid of getting a steal, many more as well).
For a pre-print of a research paper (which has been already been submitted to a journal for consideration of publication), it is not advisable to publish it in another online journal. One must wait for the response of the journal (to which paper has been submitted). If the paper is rejected by the journal then one can go ahead with its publication in another online journal.