Is this normal on Google?

Hi. I run a website that focuses on the post-secondary education sector. I have been pretty aggressive in marketing the site using Facebook and have amassed over 1,000 friends, most of whom have jobs in the career field my website focuses on. Two weeks ago, after finally getting 1,000 friends, I decided it was time to invite all 1,000 friends to "Like" my Facebook page which is directly tied into my website. In a time span of about two days, I received a little over 100 likes. I then invited all my Facebook friends to join my Google Plus page and twenty of them have started following my Google Plus page. About a week later, I was checking Google at work to see where my website was ranked and lo and behold, on one of the computers at work, it was on the first page in the number 3 spot for my fairly competitive money search term. Two minutes later, I checked it on a different computer at work that is hooked up to the same exact internet connection, and my website was ranking on page number 7 on that computer for the same exact search term. I thought this was very odd. I went home and checked to see where my website was ranking on my own laptop, and it also showed that my website was on page 7 for the same exact search term. I was not signed into my Gmail account in either instance. Now, my website has dropped all the way to page 17 in a matter of four days and this morning has jumped up to page 13.

My website went live only four months ago so, it is relatively young, although according to a back link checker I use, it has over 2,000 back links already. Is what I experienced normal? It seems like it would be almost impossible to stay on the first page of Google for a fairly competitive search term unless you are getting tens or hundreds of Facebook likes or Google +1's each day!

One last thing, I forgot to mention that may explain why my site dropped way down to page 17 after rising so rapidly. At about the time my website rose to page 7, I was dropping a bunch of back links on Pinterest by putting a link to my website URL in each picture's description. The back link anchor text was exactly the same on every picture and I probably made about five hundred back links using this same anchor text within a matter of three days. Could this have caused Google's algorithm to suddenly push my website's page rank from page 7 back down to page 17? In the past two days, I have removed all those back links that were on Pinterest and this morning, like I said earlier, my website is now listed on page 13 for the same exact search term.