To be a little more precise? Super Smash Bros. on the 3DS just sold over one million units in a single weekend. In Japan alone.
That number, like all sales numbers, may seem like an abstract figure completely devoid of meaning, but here's a little perspective: the only other 3DS games that managed to sell that amount of units and that short a period of time were Pokemon X & Y and Monster Hunter 3.
Basically, Super Smash Bros. just joined a very exclusive club. The only other members are video game franchises that Japan goes absolutely loopy for.
It's difficult to tell how these numbers will translate to Western markets, but the last two major Super Smash Bros. releases? They sold roughly to the same level in the European territory compared to Japan, but folks in the US really tend to buy the most copies of Super Smash Bros. — US sales tend to account for at least 50% of the lifetime sales of the series.
But it is worth nothing that the handheld market, per capita, is extremely strong in Japan. This may skew things a little. That being said: Super Smash Bros. on the 3DS is going to shift a lot of units. That is most likely inevitable. I'd be surprised if it doesn't become the most successful version of the game to date.