Outside of the MSC world, I have worked for 5 companies that were bought out. I also worked for a Fortune 50 company that bought out 4 other companies while I worked for them.
In a purchase/merger there is always a transition period. During the transition, the books have to be closed on the acquired company and the work in progress has to be transitioned to the new company.
Whatever payment processes were in place may or may not be continued while accounts for vendors to the old company get paid out. It was not unusual to see an electronic payment be transitioned to a check for the final payoff out of funds from the old company.
For example, the old finance guy might not be around anymore. Sometmes all the payment funds for the old vendors (that's us!) may be moved to an isolated bank account. In this case, it is faster/easier for finance to write a check to the old vendors instead of set up electronic payments that may happen only once or twice more from that acount before the acquired company's books are closed permanently.
Then the transition happens and everyone still working for the new company (whether employee or contractor) is under the new rules and new payment procedures.
My guess is this is why you got checks. They have isolated the funds to pay off the remaining FB+ shops. Once those final payments are processes, that account will be closed.
They may pull in the FB+ shoppers accounts into their system, but that is less likely as you will probably have to sign a new ICA with different terms and conditions.
Mergers and Acquisitions are always interesting and very hectic. Probably someone should have sent email to all the FB+ shoppers saying that is what would happen. But I am not surprised it didn't happen since everyone in the acquiring company is super busy with the transition.
Keep in mind, a check instead of a paypal payment, while inconvenient, is still minor compared to those employees who may lose their jobs due to "streamlining operations".