It's a really interesting question. My gut tells me that Shopify is looking for a suitor and this is facebook's way of de-risking (validating?) a potential investment.
Shopify is definitely not looking for a suitor - no way. They still have so much growth potential - that rocketship is looking to add more channels. Once on Shopify it's tough to leave - that whole switching cost.
A lot of Shopify's competitive advantage comes from being a giant, Canadian tech company. Similar to Blackberry before it, the government is extremely generous and investors price the stock accordingly. If Shopify moved to the States or was acquired by an American company I think it would take away that unique appeal.
Also the founder is an egomaniac and would never sell.