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The problem is that it forces you to break G-Rule #1 - manage your website for the visitors, not the search engines.
@littlegiant..as I stated before ( does anyone read previous posts in threads properly ? ) that still doesn't change your IP ( even if you click on nothing ) so your serps are still "contaminated" just to a lesser degree..
Frankly if 3000 more people 'like' or 'plus one' or somehow publicly indicate that they like my site better than someone else's, I think it *should* rank higher than the site who doesn't get those social signals.
And if someone gets 10,000 more likes or plus ones than I do, then they should probably outrank me.
I suspect some of those opposing it so vigorously realise that, and they're trying to keep the field free for themselves ;) (Else why would they bother to follow this thread?)
Or let's put it this way - you know what +1 stands for, so when was the last time you visited a website and clicked it? This thread is already 3 pages long, yet the +1 counter at the bottom is still at zero...
"Social signals" and +1 among them are of great advantage for high traffic websites over any new comers.