Sophie, we are banned from using facebook at work, so yes, you might be putting them in an awkward situation. I've had this happen sometimes when colleagues who I barely know seems to have invited everyone in their address book to join their personal FB page, we just have to decline automatically,. If you do go ahead - don't be offended if they don't join you, lots of companies have very strict policies about the use of social networking sites on work computers, or with work email addresses.