MySQL - Is there a theoretical limit?

Guy Kawasaki interviewed Marten Mickos. Marten was the CEO of MySQL, now he is the senior vice president of the database group within Sun.

Interesting question about MySQL scalability:

Guy: Is there a theoretical limit of MySQL in case a small business uses it and sales/transactions/whatever explode?
Marten: Every software product has its limits, but I think we have shown that MySQL can scale enormously. Google runs its entire ad system on MySQL. Nokia runs mobile phone networks on MySQL. Booking.com runs all their business transactions on MySQL. If a small business reaches those limits, it is not a small business any longer--it is an enormous global player.

I am a fan of MySQL myself.

Good to know that FaceBook also uses MySQL (with it's 65 million users), and YouTube used MySQL and Twitter uses MySQL (Scaling Twitter: Making Twitter 10000 Percent Faster).