A new milestone of sorts has been reached in the ad blocking realm. Adblock Plus, the leading ad blocking tool, has just announced that it’s just passed the 100 million marker in active installations.

An earlier milestone – 500 million downloads – was reached at the beginning of this year. That means the active user base has now doubled in less than half a year.

If these figures are accurate – and there’s little reason to think that they aren’t – it’s a pretty big deal. No longer is ad blocking an exotic functionality that’s the exclusive preserve of techies or other geeky subgroups.  It’s gone majorly mainstream.

What’s driving the ad blocking business is the ubiquity of online advertising. For many viewers, it’s nothing short of intolerable:  obtrusive, irritating, and sometimes creepy (hello, retargeting).

So once a well-functioning and reputable tool like Adblock came along, it was only a matter of time before it would take on “snowball-rolling-down-a-mountainside” proportions.

AdBlock Plus promises “annoyance-free web surfing.”  But as with most any innovation, there are one or two hitches. For Adblock Plus, it’s something called “Acceptable Ads.”

“What’s that?” you might ask. It’s a white-list program that allows certain advertisers through Adblock’s screen.  The company receives a cut of publishers’ revenues through that program.

Fundamentally, it’s how Adblock Plus makes money. But it’s also how advertisers can do an end-run around the very service Adblock provides.

AdBlock goes to great pains to “explain” its rationale and why the Acceptable Ads program makes sense for everyone.

But it isn’t difficult to see where this might end up.  Larger advertisers will see fit to exempt themselves from ad blocking by paying for the privilege of their ads being served.

Which gets us right back to where we were with advertising in the first place, doesn’t it? Pay to play.

What’s old is new again, I guess. And meanwhile, the online ads just keep coming …