This could be good or bad for Opera.
Opera has never achieved the popularity that it deserves. One of the first browsers to support mobile devices, I believe it was the first to include a phishing filter and scan downloads for malware. But its biggest setback was taking too long to support add-ons (extensions).
In my opinion, it is a much better browser than Chromium.
If Facebook bought Opera, they have the money to help it improve and promote it. They can make it better and more popular.
One of the main reason Google Chrome got so popular was Google is a large company, they put a lot of money into promoting it and bundling it with other products. Another reason was the open-source Chromium project allowing developers to make improvements and extensions.
If Facebook would do the same with Opera, I'm sure it would be a success.
However if Facebook made some wrong decisions, then it could hurt Opera.
Thanks.