Yes, quite possibly. When you’re at work, you’re supposed to be working, so online social networking on your employer's time could be grounds for initiating a disciplinary process.

Your employer should have a clear written policy on personal use of IT equipment, and should have made it clear to you what you are allowed to do (both in work and outside work), what you can expect to be punished for doing, and what sort of punishment is likely.

Your employer should have a clear disciplinary procedure that must be followed properly, including your right to be accompanied into a formal meeting by a union representative or a co-worker.