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Then I commissioned somebody to submit my site to 100 social media sites and 100 internet directories.
Now it's nowhere to be found among the top 100 for that keyword.
Can Social Media Submissions hurt rankings?
Does Google track usernames and mark them as "spammers" (e.g. spammy link builders)
This Google practice sometimes opens the door to your competitors being able to hurt your rankings, under some situations. The competitive situation isn't that simplistic, and Google does work to limit the possibility - but they also acknowledge it by saying that "There's ALMOST nothing a competitor can do to harm your ranking..."
You could have destroyed trust in your site with a bunch of crummy links, but the trust can come back with time. I've even seen cases where the site came back a bit stronger than before, but as I stopped using these sort of links some time ago I can't say if this is still the case. Google might not be so forgiving these days.
I found that as we started to submit the URL/same keywords, the keywords started to drop in SERP's from page#1 to Page #2, #3 and sometimes even #5. ( Note: All 10 keywords were ranking on page#1 for their respective URLs). I some cases the keyword rank stayed, but the URL was removed from the ranking.
However, due to the authority of the site, the ranks were all back within 2 weeks, but for me a complete NO to Social submission for sites hereon.