You're being entirely reasonable with your line of questioning. There is a Facebook operation in the UK, but the scale and reach of the issue is such that the House of Commons committee concerned wants to hear from the company's executive leadership.

Unfortunately, the BBC News article misrepresents what the select committee chairman actually said with his letter, which was to merely request Zuck's appearance before the committee. There is no suggestion in the letter itself that Zuck is being 'summoned', which would be both amusing and embarrassing.