That question should probably be added to the FAQ section under HELP at the very bottom so that a standard reply can be provided. So, a History lesson...of sorts...at least my take on history.
A long time ago (last millennium), in a basement far away (suburban Tallahassee) it was decided that if a member (Member A) of the iHub Community thought another member's (Member B's) posts were relevant (or irrelevant) and wanted to follow the postings of Member B, there should be a way for Member A to see all of Member B's posts all in one place. And so it began. All Member A had to do was "Membermark", as it was called back in the day, or "Follow" (Thanks, Twitter & Faceplant) and Member B now showed up in Member A's FAVORITE list. So now, any time Member A wanted to see the sage postings of Member B, all Member A had to do was go to his/her FAVORITES list, click on Member B's profile and voila! there were all posts made by Member B that would change Member A's life...forever. So the original theory was that Members would only mark other Members who posted of value. So you had to earn those membermarks. Therefore, the more membermarks/follows a Member has, the more prestigious that Member would be in the eyes of the iHub Community.
But Members also Membermarked/Followed peeps for a variety of reasons it turned out. Keeping tabs on other members (ie stalking), people who posted witty things, people who posted stupid things etc all became reasons to Mark/Follow other members.
So much for theory.
However, the idea that the more membermarks one has, the higher their standing in the Community, remained. So people started begging for membermarks. Even embedding links in posts that when clicked, added that Member to Member A's favorite list unbeknownst to Member A (that is my personal favorite). Then a board was set up for the sole purpose of getting membermarks. "Mark me & I will mark you" became their mantra. Then came the Private Messages - "I gave you Mark #XXX, hit me back".
So in a nutshell (finally), the reason you are getting those messages, is that other Members want to enhance their standing in the iHub Community (or at least give the appearance of an enhanced standing) by having a lot of membermarks without proving themselves. It will also clutter up your FAVORITES list