I think you have built links to fast because it's dentist your city and not a big niche like weight loss, it's local. You say you have built 500 links in 3 weeks, depending on what kind of links it is: Anchor text, PR, Relevant Content?, And more, And how you mixed these links, (Did you only do web 2.0 links, Do-Follow, No-follow, or only SB blasts, Profile Links??, Video links, Ebook links, god knows...) Always try to mix as much as possible, so it's unpredictable for google.
PLUS I think you made the mistake of sending the SB links straight to your main site. You should have "FILTERED" it through a web 2.0 site or Article Site, with the right on-page seo, good amount or quality links built to this web, and anchor text. Then from the web 2.0 page link to your main domain. I did the same that you did a few times with my local websites and google caught me.
How? I think because it can see your process one way or another, and because there is nearly no LOCAL optimizers, or let's say dentists, there could not possible be information coming in every day, and heavy links build every day (Funny enough maybe JUST SB blasts which makes it even more obvious) make it more obvious that you are trying to rank.
You should create QUALITY links (Web 2.0, Ezine articles, Etc..) with the right anchor text to your website, and SB Blast links to these web 2.0 sites. Then a extra maybe build a few extra web 2.0 sites, linkwheel, etc...
Try to remember:
Have a optimized article, pointing to your optimized web page, with the right optimized anchor text. There are a lot more factors that could be counting, but this is when your in highly competitive niches, not local.
Don't leave footprints, think like a search engine, there are even plugins that show your website like a search engine bot sees it, just try it out see how it looks. It might change your thinking a bit.
Understand the value of your competitors On-page strategies, and links.
I see it like this: Your whole competition is see through!
What do I mean? You know WHO your competition is, You know how STRONG your competition is, You know what to do to OUTRANK your competition.
You also know how many people search for the keyword, howmuch people might come to your website and howmuch you will make out of it. So funny enough, all your questions can be anwsered by simple research.
Luca