Is it permissible for a single young man to inquire about his former female colleagues, married and unmarried, via Facebook or phone after the completion of their studies and joint work?

It is permissible for a man to communicate with a woman through communication channels when there is a need, provided there is no fear of temptation (fitna) and no suspicion. However, speaking with a young woman is usually a cause for temptation, which is why jurists have strongly prohibited it. It is better to block the means [to evil] and close the door to mischief, especially for a single young man, so that he is not subjected to ill suspicion and negative consequences.