lexipixel - 11:17 pm on Aug 15, 2011 (gmt 0)
You would be promoting the use of Facebook and Twitter.. in effect advertising for them (for free).
Just check their guidelines to make sure you comply with certain things, (e.g.- Facebook general guideline #5 "You must keep sufficient space around our Brand Assets so they appear clean and uncluttered.", etc).
(Facebook) Brand Permissions Center:
Guidelines for Use of the Twitter Trademark: