Did Facebook cross the line? I'd rather say that they made this move inappropriately. The difference is that I do agree with Facebook that something very similar to this program could work, but it needs to be presented properly and at least start with more-than-ample opt-out options.
Facebook's right that this is a huge marketing/advertising opportunity and that their customers could indeed benefit from this. But they didn't properly sell those benefits to those users.
Targeted ads based on private history are dicey stuff. Sharing private purchases with everyone on a friends' list is an order of magnitude touchier.
It needs restrictions (for gift giving or other sensitive situations), but a modified version of this might work well, say one year from now. The prospect of having a surprise birthday gift ruined is one thing. What about the married customer whose bride is alerted that the customer is purchasing lots of red roses or diamond earings, that the spouse never sees? Lots of dangerous situations potentially there....
Giftcard trading site company Leverage, for example, is trying to push the envelope with advertisers allowed to pitch customers based on their giftcard holdings along with self-reported "demographic, psychographic, gift occasions and travel plans."
If we're ready for psychographically-selected ads (as opposed to what we typically see with GoogleAds, which would be more accurately labeled psychotically-selected ads), it's hard to argue that Facebook overreached. But a little more selling of the benefits and permission-getting might have made a world of difference.