Well, maybe the low quantity of your poems will make the quality even better. If someone on Tumblr steals your writing, you email support@tumblr.com and provide them with examples of your work with the dates and links to them, and then examples of the plagiarized work with dates and links to those.
If you really have been plagiarized, Support can remove the plagiarist’s blog for you. (Trust me, I know how this works.)
In regards to online publishing-you could always keep a journal of poems that you don’t post online and save them to submit to contests.
:)