This is roughly analogous to being asked to testify before a congressional committee in the US. It doesn't matter who you are in the business world: if people at this level of government invite you, you show up.

If you have an extremely good reason not to attend, maybe you can get away with sending sincere apologies and an adequate delegate to appear on your behalf, at least in the short term. Otherwise, while it might not be possible for the committee to compel Zuckerberg to appear before them in person by law, failing to do so would almost certainly have repercussions for Facebook's continued success in the UK.