I am not a lawyer, but I am in my last year of law school, and while the NCAA rules aren't statutes, they are (for our purposes) "laws" of a sort. It is very possible under those rules for fans to get their alma mater in trouble and cause a recruiting violation.
For example, fans who chant an unsigned recruit's name at an event that the recruit is attending would be a violation of rule 13.10.5. You can also get your school in trouble for holding up a sign at those events that mentions the recruit.
Fans at NC State who created a Facebook fan page for an unsigned recruit, were told by the school that they had to take the page down. If it remained active, the NCAA had advised NC State that the school would be penalized for the violation. A number of fans have invoked the wrath of the NCAA on their school on the internet, and even though our actions may be outside the school's control, the NCAA remains uninterested in adjusting.
Current students and "boosters" are more likely to cause a secondary violation when they contact high school athletes on social networks. Nonetheless, the NCAA rules state that anyone who is known or should have been known by a member of a school's athletic administration to be "assisting in the recruitment of prospective student-athletes" is considered a representative of that institution's athletic interests. This means that almost any fan with a Twitter or Facebook account could cause a violation.
Direct contact with recruits is only allowed between coaches/staff and the athlete via Fax or email (Twitter's "direct messages" qualify as email).
As a Villanova fan with knowledge of a potential recruit's Twitter account, you should feel free to "follow" these players. You can probably "retweet" their comments as well. However, any "pitch" attached to the retweet as a comment, or any reply directed at them, or mention of them in a "persuasive" manner could get the program in trouble. Please do NOT message (direct or otherwise) any unsigned recruit on the internet.
If an unsigned recruit follows your Twitter account, feel free to speak highly of Villanova. Talk about how awesome it is to be a Villanova basketball player. Say whatever you want, just leave out the player's names and DEFINITELY don't include the "@username" in the tweet.
One secondary violation will probably not result in much for the school, but multiple/repeated violations are considered a "major violation" under Bylaw 19.02.2.2. Intentional contact with a recruit is a Level I secondary violation that Villanova would be required to report to the NCAA immediately (unlike a Level II secondary violation which are accidental and reported quarterly).
Penalties from these sort of recruiting violations could result in a ban on the coaching staff contacting the recruit for a certain period of time. When a player like JayVaughn Pinkston is on the verge of announcing his decision, it could be absolutely apocalyptic to Villanova if Jay Wright is unable to contact him for two weeks (for example).
Update: Just to be clear - this applies for any recruit, even a signed recruit under a National Letter of Intent. The rules don't relax until the day a recruit enrolls in classes. At Villanova, that is normally in the first or second summer-session. That may vary by school or player.