But I expect they did thank you. When you had the conversation giving background info on your firm, or told them about a job opportunity, didn't the conversation simply end with them saying something like 'thanks for the info'? Or if you emailed the info to them, did t they reply to say they had received the info and express appreciation?
I didn't think I would be expecting an extra, later show of appreciation necessarily, but I would be surprised if you gave them the helpful info and it was met with total blankness and no acknowledgement. I think you are looking for more than a basic, passing thank you though.
And TBH, if this was all you received, it doesn't mean they took you for a mug, or were unappreciative. Doing a kind, helpful thing for someone never makes you a mug, especially of the pretty basic thing you describe.
TBH, I think you are perhaps over-rating the help you gave. Yes, you gave them some info and yes, perhaps in one case it took a bit of time to do so......well, those things are what people do for each other and aren't really a huge big deal. They definitely deserve a 'thankyou' but they are simply human kindnesses, not huge acts of generosity. To expect gifts or to be taken out for coffee or whatever, suggests to me that your actions were perhaps not usual for you, if you feel you need such acknowledgement of them.
I'm sure those people know you helped them and if the time comes when they can help you out, they will, again, without it being a big deal or without expectation of return.
Don't worry about being a mug or decide to do less helping out. On the contrary, look for more opportunities to do it. Do it cheerfully, as a matter of course and without expectation of reward. Let it become second nature to you,rather than a big deal. and when this happens, I just suspect that suddenly you'll find there are all kinds of kindnesses happening to you too - not just because people feel they owe it to you, but because people actually like those who are genuinely giving and unselfish.