I recently came across an old college "friend" at an industry event. I really consider him more of an acquaintance since we only saw each other due to a mutual friend. Furthermore he had declined my friend request on Facebook some years ago (face palm, I know, but it really does say something) and went out of his way to not include me in any hangouts with other friends.
To keep to the point he had asked me for my mobile number to "stay in touch" after a bit of chit chat, I really didn't want to give it to him but could come up with no reason. What could I possibly say without sounding hostile? I also didn't want to come across as bitter. We both had each others mobile numbers years before (say 5). But I really want nothing to do with this guy as he is just an opportunist, not to brag but I'm almost certain he just wants to reconnect to advance his career (perhaps say referral) since I have mid level position at a well known company in the area.
Which brings me to the question, how DO you decline giving a colleague or acquaintance that you have known for a number of years your mobile number? Random people I would say you could contact me on this email or feel free to add me on LinkedIn.